feat(thegraph): serialize and deserialize subscription extra auth token claims

[LNSD]

This follow-up PR extends the Subscriptions Auth token with support for serialization and deserialization of the "extra" claims:

• Added support for serializing and deserializing the extra claims (e.g., allowed_domains, allowed_deployments, etc.).
• Renamed the chain_id field into chain following @Theodus suggestion in the previous PR (feat(thegraph): add support for subscription payments ticket #60 (comment)).
• Added some "getter" methods to provide a unified fields access API following the already existing user() method.
• Extended the test coverage.

Note

The naming of the chain_id field is an API-breaking change. Upgrading the dependency will require changes in the code using this crate.

• Instead of relying on the struct fields, one should use the new claims field accessor functions, i.e., use either claims.chain() or claims.chain_id() instead of accessing the claims.chain field directly.

This is a follow-up PR of #60

[Theodus]

LGTM

[Theodus]

ooh, fancy