[HttpKernel][FrameworkBundle] SSI support

| Q             | A
| ------------- | ---
| Bug fix?      | No
| New feature?  | Yes
| BC breaks?    | No
| Deprecations? | No
| Tests pass?   | Yes
| Fixed tickets | #9419 (, #10684)
| License       | MIT

It does not support comments, or alternative URIs, or "continue" in case of errors. Maybe I can workaround that, but I've decided to left it out for this PR. Especially as far as I can see a "alternative URIs"-hack would _always_ lead to two requests, even if it's not needed.