[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 (see #6829).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

### about trusted proxies

The `hinclude` strategy should respond to any client IP but the `esi` strategy should only respond to configured IP addresses. This suggests checking IP should happen downstream of the listener…

### about config.yml

I would also like to rework the `esi` and `router_proxy` sections of `config.yml` info one `sub_request` section that looks something like this…

```yml
framework:
    # ...
    sub_request:
        listener:
            enabled: true
            prefix: /_proxy
        esi:
            enabled: true
            trusted_ips: "%kernel.trusted_proxies%"
        hinclude:
            enabled: true
```

I've changed `path` to `prefix` here so the URI could explicitly tell the listener which strategy to call up, allowing that strategy to check the IP address if necessary. For example, `/_proxy/esi?…` would be caught by the listener, which would then look for an `esi` strategy, which would check the client IP. A similar request for `/_proxy/hinclude?…` would call up the `hinclude` strategy, which would not check the client IP.