Enhancement: Async generated content in response body

[geeshta]

Summary

I've added #2359 because it seemed like an easy enough change to implement but as @euri10 pointed out, it might not really bring much if anything.
However the conversation around it pointed out the issue that there isn't a good way to await response body content from async sources - which could be async generated templates like what Jinja supports or files read asynchronously.

@provinzkraut and @peterschutt suggested some solutions in the discussion from the PR so this issue proposes implementing the proper abstraction. So that async content can be served withou breaking separation of concerns and adding this logic to ASGIResponse classes. Maybe it could potentially help with #1250 ? Am not sure though. However if this gets through, #2359 could be reimplemented in a proper way.

Basic Example

No response

Drawbacks and Impact

No response

Unresolved questions

Is this even worth pursuing? Jinja rendering and file serving currently both work ok without this.

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