config: introduce ConfigVars

A new config API is introduced, and ~all of the codebase is adapted to it.
The old API is kept but mainly only for dynamic usage where its extra flexibility is needed.

Using examples, the old config API looked this:
```
>>> config.get("request_expiry", 86400)
604800
>>> config.set_key("request_expiry", 86400)
>>>
```

The new config API instead:
```
>>> config.WALLET_PAYREQ_EXPIRY_SECONDS
604800
>>> config.WALLET_PAYREQ_EXPIRY_SECONDS = 86400
>>>
```

The old API operated on arbitrary string keys, the new one uses
a static ~enum-like list of variables.

With the new API:
- there is a single centralised list of config variables, as opposed to
  these being scattered all over
- no more duplication of default values (in the getters)
- there is now some (minimal for now) type-validation/conversion for
  the config values

closes https://github.com/spesmilo/electrum/pull/5640
closes https://github.com/spesmilo/electrum/pull/5649

Note: there is yet a third API added here, for certain niche/abstract use-cases,
where we need a reference to the config variable itself.
It should only be used when needed:
```
>>> var = config.cv.WALLET_PAYREQ_EXPIRY_SECONDS
>>> var
<ConfigVarWithConfig key='request_expiry'>
>>> var.get()
604800
>>> var.set(3600)
>>> var.get_default_value()
86400
>>> var.is_set()
True
>>> var.is_modifiable()
True
```

electrum/paymentrequest.py

@@ -400,10 +400,10 @@ def verify_cert_chain(chain):
     return x509_chain[0], ca
 
 
-def check_ssl_config(config):
+def check_ssl_config(config: 'SimpleConfig'):
     from . import pem
-    key_path = config.get('ssl_keyfile')
-    cert_path = config.get('ssl_certfile')
+    key_path = config.SSL_KEYFILE_PATH
+    cert_path = config.SSL_CERTFILE_PATH
     with open(key_path, 'r', encoding='utf-8') as f:
         params = pem.parse_private_key(f.read())
     with open(cert_path, 'r', encoding='utf-8') as f:
@@ -453,8 +453,8 @@ def serialize_request(req):  # FIXME this is broken
 
 def make_request(config: 'SimpleConfig', req: 'Invoice'):
     pr = make_unsigned_request(req)
-    key_path = config.get('ssl_keyfile')
-    cert_path = config.get('ssl_certfile')
+    key_path = config.SSL_KEYFILE_PATH
+    cert_path = config.SSL_CERTFILE_PATH
     if key_path and cert_path:
         sign_request_with_x509(pr, key_path, cert_path)
     return pr