Convert oembed image endpoint to async

[sarayourfriend]

Problem

The oembed image endpoint sometimes makes an outbound GET request to retrieve image dimensions.

openverse/api/api/views/image_views.py

Line 120 in b91cdc3

image_file = requests.get(image.url, headers=self.OEMBED_HEADERS)

This can be converted to async to avoid blocking the worker on this outbound request.

Description

1. Update the oembed endpoint to async def.
2. Wrap get_object_or_404 in sync_to_async (it calls the synchronous Django ORM methods)
3. Swap requests.get with aiohttp.get (retrieve aiohttp session using get_aiohttp_session)

That should be it! All tests should continue to pass. There may need to be minor changes to accommodate the requests to aiohttp switch.