Add support for FEP-844e: Capability discovery

[sij411]

Summary

Add support for FEP-844e so applications built with Fedify can advertise and discover their ActivityPub capabilities.

Implementation details

• Add typed vocabulary support for the unordered implements set on Application, including capability objects with a required href and an optional name.
• Serialize and deserialize the https://w3id.org/fep/844e context correctly.
• Add interoperability tests based on the FEP examples.
• Document how applications can advertise capabilities directly, through an actor's generator, and through a server-level actor discovered with FEP-d556.
• Update FEDERATION.md once support is complete.

Server-level discovery depends on FEP-d556. Fedify's existing WebFinger APIs appear capable of expressing this flow, but the implementation should verify that both publishing and discovering the required https://www.w3.org/ns/activitystreams#Service link work correctly. Any missing general FEP-d556 behaviour can be split into a dependency issue if needed.