remove create_sync_endpoint, run sync functions in background thread

[geospatial-jeff]

Description:
Remove some code duplication by removing create_sync_endpoint. Instead wrap synchronous function calls in a decorator which executes them asynchronously through a background thread using starlette.concurrency.run_in_threadpool. This means all endpoint handlers used by async or sync backends will be executed asynchronously.

FastAPI also executes synchronous endpoint handlers in background threads, so this should behave the same as before but with one less code path.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.