[WebProfilerBundle] Show AJAX requests in the symfony profiler toolbar

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Adds AJAX requests in the web debug toolbar.
See #8896 for the original discussion.