Add tests for auth flow in custom feeds

[MarshalX]

Finally, it's possible! 🎉 The long story of achieving it:

• Implement atproto identity (DID and Handle resolvers) and DID Document #219
• Implement Service JWT #225
• Add multibase encode and decode python-libipld#3
• Implement AtprotoData and DID key formatting and parsing #227
• Implement signature verification #232

In total about 4k additions with docs and tests and about 25 hours of coding

Documentation is deployed already https://atproto.blue/

[MarshalX]

Thanks again for help with fixing signature verification to @DavidBuchanan314 and @snarfed!

@emilyhunt Hi, be prepared to implement user-related features in your feeds! It's coming in the next release.

@DXsmiley Hi, I saw you already implemented something around auth in custom feeds. Maybe it's a good time to switch to SDK`s set of tools.

Upd. released in v0.0.37