Add LitePub relay implementation

[sij411]

Summary

This PR adds a LitePub-compatible ActivityPub relay implementation to the
@fedify/relay package, complementing the existing Mastodon relay implementation.

Related Issue

#359

Changes

• Added LitePubRelay class in packages/relay/src/relay.ts:321-585 that implements
  the LitePub relay protocol
• Implemented LitePub-specific activity handling:
  • Wraps incoming Create, Update, Move, and Delete activities in Announce
    activities before forwarding
  • Uses Application actor type instead of Mastodon's Service type
  • Sends announcements to PUBLIC_COLLECTION for broader visibility
• Added comprehensive test suite for LitePubRelay in
  packages/relay/src/relay.test.ts:455-832 covering:
  • Relay initialization and configuration
  • Actor endpoint behavior (Application type)
  • Followers collection management
  • Subscription handler integration
  • Follow/Undo activity processing
  • Multiple follower support

Benefits

• Protocol compatibility: Enables Fedify relay instances to work with
  LitePub-compatible fediverse software, which expects activities to be wrapped in
  Announce rather than directly forwarded
• Broader reach: Using PUBLIC_COLLECTION as the audience for announces ensures
  better visibility across the fediverse
• Developer choice: Developers can now choose between Mastodon-style
  (MastodonRelay) and LitePub-style (LitePubRelay) relay implementations based on
  their target ecosystem
• Consistent API: Both relay implementations share the same Relay interface, making
  it easy to switch between protocols

Checklist

• Did you add a changelog entry to the CHANGES.md?
• Did you write some relevant docs about this change (if it's a new feature)?
• Did you write a regression test to reproduce the bug (if it's a bug fix)?
• Did you write some tests for this change (if it's a new feature)?
• Did you run deno task test-all on your machine?

Additional Notes

Key differences between LitePub and Mastodon relay protocols:

1. Actor type: LitePub uses Application, Mastodon uses Service
2. Activity forwarding:
   - LitePub wraps activities in Announce objects with relay as actor
   - Mastodon forwards activities directly using ctx.forwardActivity()
3. Audience: LitePub sends to PUBLIC_COLLECTION for maximum visibility

The implementation follows the same subscription management and key pair handling
as MastodonRelay, ensuring consistent behavior for core relay functionality like
follower management and subscription approvals.