[BrowserKit] add a way to switch to ajax for one request

[javiereguiluz]

Let's wait a bit before documenting this because someone (🙄) has asked to rethink the proposed implementation: symfony/symfony#24778 (comment)

symfony/symfony#24778

[Simperfit]

I'll document it when the new PR is merged.

[javiereguiluz]

This is now ready and the new implementation is symfony/symfony#26381