Implement all API endpoints of dart-nextcloud

[provokateurin]

See https://github.com/jld3103/dart-nextcloud/tree/master/lib/src. There will definitely be breaking changes, but we need all endpoints so it's possible to migrate at all.

• core: autocomplete user (https://github.com/jld3103/dart-nextcloud/blob/0cc8e1df7ef8b4b4407d4f8c5271453fd6149e24/lib/src/autocomplete/client.dart#L19)
• core: avatar (https://github.com/jld3103/dart-nextcloud/blob/0cc8e1df7ef8b4b4407d4f8c5271453fd6149e24/lib/src/avatar/client.dart#L18)
• core: preview by file id (https://github.com/jld3103/dart-nextcloud/blob/0cc8e1df7ef8b4b4407d4f8c5271453fd6149e24/lib/src/preview/client.dart#L59)
• core: thumbnail (https://github.com/jld3103/dart-nextcloud/blob/0cc8e1df7ef8b4b4407d4f8c5271453fd6149e24/lib/src/preview/client.dart#L145)
• files_sharing: sharees (https://github.com/jld3103/dart-nextcloud/blob/0cc8e1df7ef8b4b4407d4f8c5271453fd6149e24/lib/src/sharees/client.dart)
• talk (https://github.com/jld3103/dart-nextcloud/tree/0cc8e1df7ef8b4b4407d4f8c5271453fd6149e24/lib/src/talk)

[sonigeez]

what's update on talk APIs?

[provokateurin]

They are work in progress: https://github.com/provokateurin/nextcloud-neon/blob/feature/openapi-demo/specs/spreed.json
I assume they will be available in about two months, but no promises given ;)

[provokateurin]

@Leptopoda this should be promoted to the 1.0 release milestone as we want to publish the package then and it would be bad if we wouldn't have all APIs implemented in the new version, right? Not sure if Talk will make the cut though, that could take some more time. It doesn't really matter though because the current package only implements an old version of the Talk API which we are never going to implement anyway.

[provokateurin]

The missing APIs would all be ticked off once we have #417. I would just wait for that otherwise implementing the missing endpoints would be wasted work.