Support auth for Lemmy 0.19

[micahmo]

This PR introduces support for authenticating with Lemmy 0.19 instances. Note that I haven't tested anything beyond auth, although basic things seem to work fine.

Preface: For some reason, dart run build_runner build changed a ton of stuff (like adding Impl in a bunch of places to the generated code). I'm not sure if is because we updated build_runner in 2476ef6, and didn't regenerate, but I hope it's ok. In general, I think we can ignore .freezed.dart and .g.dart files anyway.

Here is a summary of changes and references to find amongst all the auto-generated code.

1. Make the person admin field optional, so that we can handle its absence.
   • Change: 62ddbb7#diff-27a77cc09ea7733233a3579b24d13c74c63c5c12a35ee6c71ac4ee59b46a001e
   • API Ref: LemmyNet/lemmy-js-client@973b255#diff-f4e889928e27357491c23f38f4ff2ee189af3ebdacc91c6560a0f29c8df17b7cL20
2. Add auth token to headers.
   • This was done by adding a new abstract class with the auth property (code). All existing API methods with auth were updated to implement this class. Then in the invocation code, we can simply check whether the request contains auth, and pass that as a header (in addition to the existing query params) (code). I believe this approach has the following advantages.
     • We can support both methods of auth simultaneously (as recommended by Lemmy devs).
     • We can use the existing pattern of setting auth in the request object (and don't have to change any of the API models).

Related to thunder-app/thunder#762.

Demo

scrcpy_lFuPrUUco7.mp4

[hjiangsu]

LGTM! Thanks again for taking the initial steps to add support for 0.19.x. We can do some more testing on this in the nightlies to find any issues that come up