Support Mastodon API-compatible login to external clients.

[RokeJulianLockhart]

Is your feature request related to a problem? Please describe.
I'm unable to use Bookwyrm in any existent native clients.

Describe the solution you'd like
Support at least the minimum Mastodon API compatibility necessary to permit login to clients which support it, like Fedilab.

Describe alternatives you've considered
#785 (comment)

Additional context
https://codeberg.org/tom79/Fedilab/issues/1027#issuecomment-1528213

[rabc]

There is already an issue opened regarding API #785

[RokeJulianLockhart]

#3291 (comment)

@rabc, albeit a Bookwyrm API, so not within the purview of this issue. However, I'll add it as a potential alternative.