Content renderer simplification

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#2179

Todo:

  - [x] submit a PR for documentation update

The first commit makes the Request required when dealing with rendering strategies (see the commit why this was a bad idea to make it optional in the first place).

The second commit removes the need for a proxy route and replaces it with the same system we have in the security component.

The third commit makes the proxy path configurable (default to `/_proxy`).

This PR has been triggered by a discussion on #6791.