Async Support

[bigcat88]

Changes proposed in this pull request:

• AsyncNextcloud and AsyncNextcloudApp are here!
• Reworked set_handlers to allow use of async handlers.
• set_handlers: denied defining init_handler and models_to_fetch at the same time, as huggingface_hub does not support async.

If you want you can provide your own sync or async init_handler callback and do in it what you want.
If you need only automatic AI models to download(probably all will use this behaviour), nc_py_api will handle on its own.

What is currently missing: caldav async for AsyncNC classes.
If you need to use CalDAV use standard synchronous classes.

After this PR there will one more separate PR where models_to_fetch and models_download_params will be united in a single more flexed parameter.

[codecov]

Codecov Report

Merging #181 (2fe5d10) into main (52b48f1) will increase coverage by 0.08%.
The diff coverage is 99.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   99.84%   99.93%   +0.08%     
==========================================
  Files          36       37       +1     
  Lines        3260     4497    +1237     
==========================================
+ Hits         3255     4494    +1239     
+ Misses          5        3       -2     

Files	Coverage Δ
nc_py_api/__init__.py	100.00% <100.00%> (ø)
nc_py_api/_preferences.py	100.00% <100.00%> (ø)
nc_py_api/_preferences_ex.py	100.00% <100.00%> (ø)
nc_py_api/_session.py	100.00% <100.00%> (+0.86%)	⬆️
nc_py_api/_talk_api.py	100.00% <100.00%> (ø)
nc_py_api/activity.py	100.00% <100.00%> (ø)
nc_py_api/apps.py	100.00% <100.00%> (ø)
nc_py_api/ex_app/__init__.py	100.00% <100.00%> (ø)
nc_py_api/ex_app/integration_fastapi.py	100.00% <100.00%> (ø)
nc_py_api/ex_app/ui/files_actions.py	100.00% <100.00%> (ø)
... and 14 more