xlwings Server: respect the sync/async definition of custom functions

[fzumstein]

Currently, all custom functions run on an async server endpoint, even if they are defined as a sync function, which will block the event loop.
Instead, sync functions should behave like a sync fastapi endpoint, possibly via run_in_threadpool: tiangolo/fastapi#10768
Might need to be solved on the backend implementation though to not make this framework-dependent.