CW-53: Added a runtime metadata check

[Marcin-Radecki]

Description

This PR adds a runtime metadata check, which consists of two phases:

1. Build subxt using large self-hosted runner (1 minute)
2. Run subxt codegen on runtime built from main (used in e2e tests - short session) (1 minute)
   This check fails if someone forgets to regenerate the aleph-runtime static metadata file.

There's no if condition when this check is supposed to run as if practically it does not make sense to check, e.g., if runtime code changed or aleph pallet code changed in git, as then we'd need to check every aleph-runtime dependency, etc. This check is lightweight (2-3 minutes, and runs in parallel with the longest check excluded packages).

Also, this PR consists of changes to the Aleph::NextAuthorities hash. This hash is what subtext thinks the hash of NextAuthorities metadata is. Normally, this hash does not include any of what is stored as a storage value, but only data such as pallet name, storage function name, storage value type (Option<T> or T), etc. Also, maybe a bit surprisingly, it includes default value of given type into that subxt hash. Now, NextAuthorities storage value type is Vec<AccountId>, and normally default is vec![]. However, we do have a default initializer DefaultNextAuthorities which, turns out, is used by subxt codegen when generating hash for NextAuthorities. When DefaultNextAuthorities, static runtime metadata generated by subxt is always the same in CI. DefaultNextAuthorities is not required in pallet aleph, as initialization of NextAuthorities is done in on_genesis_session.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Testing

• Runtime changed but no metadata regenerated - fail - OK
• Runtime changed and metadata regenerated - pass - OK

[pmikolajczyk41]

well done! lgtm