Lexrpc

Notable changes

* Add array type support.
* Add support for non-JSON input and output encodings.
* Add `subscription` method type support over websockets.
* Add `headers` kwarg to `Client` constructor.
* Add new `Server.register` method for manually registering handlers.
* Bug fix for server `method` decorator.

Bluesky's Lexicon design and schema handling is still actively changing, so this is an interim release. It generally supports the current lexicon design, but not full schema validation yet. I'm not yet trying to fast follow the changes too closely; as they settle down and stabilize, I'll put more effort into matching and fully implementing them. Stay tuned!

_Breaking changes:_

* Fully migrate to [new lexicon format](https://github.com/snarfed/atproto/commit/63b9873bb1699b6bce54e7a8d3db2fcbd2cfc5ab). Original format is no longer supported.

Initial release!

Tested interoperability with the `lexicon`, `xprc`, and `xrpc-server` packages in [bluesky-social/atproto](https://github.com/bluesky-social/atproto). Lexicon and XRPC themselves are still very early and under active development; caveat hacker!