[HttpKernel] renamed sub-request classes

The name "HTTP content renderer" was too vague for me, so I renamed things to jive with the concept of sub-requests, which is more specific. I've also renamed the "default" strategy to "inline," which is more descriptive.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~