register_output_renderer render callback should be optionally awaitable

[simonw]

In #581 I made a bunch of improvements to this, including making datasette available to it so it could execute queries.

But... it needs to be able to await in order to do that. Which means it should be optionally-awaitable.

Original idea here: #645 (comment)

[simonw]

Example code from elsewhere:

datasette/datasette/views/base.py

Lines 416 to 420 in 52c4387

	extras = {}
	if callable(extra_template_data):
	extras = extra_template_data()
	if asyncio.iscoroutine(extras):
	extras = await extras