[HttpKernel] refactored the HTTP content renderer to make it easier to extend

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | kinda
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This makes the StreamedResponse logic reusable for other strategies and it also makes the RenderingStrategy interface less fuzzy about its contract.

That also makes features like #4470 easier to implement from the outside.