Use run_checks_async with Fastapi (or allow customization through configuration)

[grahamalama]

python-dockerflow/src/dockerflow/fastapi/views.py

Lines 19 to 21 in 4c5ae34

	check_results = checks.run_checks(
	checks.get_checks().items(),
	)

This makes it so that you can't register async checks in a FastAPI app. We should

• flip this to run_checks_async
  and/ or
• allow users to customize which method they'd like to use to run checks
  and/or
• introspect the registered checks and determine which function to use (if any of the registered checks are async, use run_checks_async