Test against Bluesky's sandbox BGS

[snarfed]

Bluesky team's federation sandbox is up! More on #federation-sandbox on the Bluesky API Touchers Discord. Example third party PDS that's sucessfully federating: https://github.com/DavidBuchanan314/picopds. Here's what we need to do:

• stand up a live PDS
• Implement all required XRPCs:
  • com.atproto.xrpc.subscribeRepos
    • websockets setup
  • com.atproto.xrpc.getRepo
  • com.atproto.xrpc.getCheckout
  • com.atproto.identity.resolveHandle
• Proxy all other XRPCs through to the appview, including resolveHandle and app.bsky.*
• Create and use a did:plc in the sandbox.
• Use the "low-S" variant for P-256 signatures. Looks like this isn't in pycryptodome, ugh. ATProto docs, BIP-62, discussion, feature request. Evidently this is doable as a transform on a decoded (not bytes) signature itself.
• Migrate to the v3 repo data model #7
• Construct real JWT to auth from PDS to appview? These are required for...timeline etc XRPCs? Can I skip this? (Here's how picopds does it.)
  • ...and use it

[snarfed]

Getting close! Switched from did:web to did:plc, generated did:plc:wchma7xpbrlf6wbvlhkvonkw with handle arroba1.snarfed.org, set up that handle's DNS, and created it on the federation sandbox PLC server.

[snarfed]

Lots of interop testing later...it's alive, it's alive!!! https://bsky.app/profile/arroba5.snarfed.org on the sandbox