Instantiate XrpcClient from an OAuthAgent

[matthieusieben]

• @atproto/api's Agent class can now be instantiated using a SessionManager interface (new Agent(sessMgr))
• @atproto/client's OAuthSession implements the SessionManager interface meaning that we can do new Agent(oauthSession)
• @atproto/api's AtpAgent works exactly as before. A comment was added to say that is will soon become deprecated as it adds no value anymore.
• @atproto/api now exposes an CredentialSession class that can be used to instantiate both new Agent(credSession) as-well-as new XrpcClient(credSession, myLex)

Potentially breaking changes:

• Agent is no longer abstract, and its constructor signature has changed. This means that any implementation extending that class will be broken. Since this abstract class was release very recently, the impact should be minimal.

• Agent no longer extends AtpBaseClient (this is an implementation detail so should no really impact anything)

• @atproto/oauth-client* no longer expose an agent, but a session instead. Adapting to this is very straight forward (new Agent(session)).

• OAuthAgent was renamed into OAuthSession and some method signatures where changed

[matthieusieben]

Hey @devinivy, thank you for the review. I moved the OAuthAtpAgent class into the api agent. This reverses the dependency between @atproto/api and @atproto/oauth-client but that extra dependency in api is only needed for typing purposes and should not impact the bundle size.

With this change, the @atproto/api is no longer needed in order to instantiate custom @atproto/xrpc clients.

[matthieusieben]

This whole PR introduces a breaking change in the oauth-client* packages but I think it's fine since those were not released yet (will impact ozone but no big deal).

cc @foysalit

[matthieusieben]

Thanks @surfdude29 for your careful review <3

[surfdude29]

Thanks @surfdude29 for your careful review <3

No worries, happy to help :)