Stop including all invoice r_tags in legacy trampoline onion

This change modifies create_trampoline_onion to only include as many
available r_tags as there is space left in the trampoline onion payload.

Previously we tried to include all passed invoice r_tags of legacy
trampoline payments into the payload which caused an user facing
exception and payment failure as the onion can only store a max of 400
bytes.
A single, single hop r_tag is around 52 bytes and the payload
without r_tags is already at ~280 bytes. So usually there is enough
space for 2 r_tags.
The implementation shuffles the r_tags on each call
so the payment will try different route hints on the attempts (fee level
increase or user retry).

I have logged the following byte sizes of the trampoline onion with a 2
trampoline onion hop and changing amounts of r_tags:

3 rtags:
payload size [0]: 113 (hop size: 81)
payload size [1]: 440 (hop size: 295) ( 52 bytes/rtag )
payload size [2]: 550 (hop size: 78)

2 rtags:
payload size [0]: 113 (hop size: 81)
payload size [1]: 386 (hop size: 241) ( 52 bytes/rtag )
payload size [2]: 496 (hop size: 78)

1 rtag:
payload size [0]: 113 (hop size: 81)
payload size [1]: 334 (hop size: 189) ( 52 bytes/rtag )
payload size [2]: 444 (hop size: 78)

0 rtags:
payload size [0]: 113 (hop size: 81)
payload size [1]: 282 (hop size: 137)
payload size [2]: 392 (hop size: 78)

As can be seen in the data, using 2 trampoline hops there is not enough
space for even a single r_tag which is why this option is being removed
too.

tests/test_lnrouter.py

@@ -4,19 +4,22 @@ import tempfile
 import shutil
 import asyncio
 from typing import Optional
+from os import urandom
 
 from electrum import util
 from electrum.channel_db import NodeInfo
 from electrum.onion_message import is_onion_message_node
+from electrum.trampoline import create_trampoline_onion
 from electrum.util import bfh
 from electrum.lnutil import ShortChannelID, LnFeatures
 from electrum.lnonion import (OnionHopsDataSingle, new_onion_packet,
                               process_onion_packet, _decode_onion_error, decode_onion_error,
-                              OnionFailureCode, OnionPacket)
+                              OnionFailureCode)
 from electrum import bitcoin, lnrouter
 from electrum.constants import BitcoinTestnet
 from electrum.simple_config import SimpleConfig
-from electrum.lnrouter import PathEdge, LiquidityHintMgr, DEFAULT_PENALTY_PROPORTIONAL_MILLIONTH, DEFAULT_PENALTY_BASE_MSAT, fee_for_edge_msat
+from electrum.lnrouter import (PathEdge, LiquidityHintMgr, DEFAULT_PENALTY_PROPORTIONAL_MILLIONTH,
+                               DEFAULT_PENALTY_BASE_MSAT, fee_for_edge_msat, LNPaymentTRoute, TrampolineEdge)
 
 from . import ElectrumTestCase
 from .test_bitcoin import needs_test_with_all_chacha20_implementations
@@ -450,6 +453,47 @@ class Test_LNRouter(ElectrumTestCase):
             self.assertEqual(hops_data[i].to_bytes(), processed_packet.hop_data.to_bytes())
             packet = processed_packet.next_packet
 
+    def test_create_legacy_trampoline_onion_multiple_rtags(self):
+        """Test to verify we don't overfill the trampoline onion with r_tags if there are more tags than available space"""
+        dummy_route: LNPaymentTRoute = [
+            TrampolineEdge(
+                invoice_routing_info=[
+                    bfh("010305061295fa30847df41ae6ee809b560e78d65c2a7337a41c725ea3920b65e08a03b62b00003a0002000003e8000000010050"),
+                    bfh("01037414fe3dcfedc4a0a0e153205d9a973af5096d1cd1c8c53d07ed12d7dd966f19f424000000000020000003e8000008ca0050"),
+                    bfh("01038550162fa86287884a6a052471934abb5cb261c5a2b15386df8104d3c7bcb85dddd92ee1898ee15c000003e8000000010090"),
+                    bfh("010244bb7ba2392ab2d493ad04ad4afcd482ca44a2bfe5b42bcc830bfe00e5b08082f424000000000029000003e8000008ca0050")
+                ],
+                invoice_features=LnFeatures.VAR_ONION_REQ | LnFeatures.PAYMENT_SECRET_REQ | LnFeatures.BASIC_MPP_OPT,
+                short_channel_id=ShortChannelID.from_str("0x0x0"),
+                start_node=node('a'),
+                end_node=node('b'),
+                fee_base_msat=0,
+                fee_proportional_millionths=0,
+                cltv_delta=0,
+                node_features=0
+            ),
+            TrampolineEdge(
+                invoice_routing_info=[],
+                invoice_features=None,
+                short_channel_id=ShortChannelID.from_str("0x0x0"),
+                start_node=node('b'),
+                end_node=node('c'),
+                fee_base_msat=0,
+                fee_proportional_millionths=0,
+                cltv_delta=0,
+                node_features=0
+            ),
+        ]
+        # create a trampoline onion, this shouldn't raise InvalidPayloadSize
+        create_trampoline_onion(
+            route=dummy_route,
+            amount_msat=0,
+            final_cltv_abs=0,
+            total_msat=0,
+            payment_hash=urandom(32),
+            payment_secret=urandom(32),
+        )
+
     @needs_test_with_all_chacha20_implementations
     def test_decode_onion_error(self):
         # test vector from bolt-04