[HttpClient] Add portable HTTP/2 implementation based on Amp's HTTP cli…

[fabpot]

Q	A
Feature PR	symfony/symfony#35115
PR author(s)	@nicolas-grekas
Merged in	5.1-dev

[wouterj]

@nicolas-grekas can you maybe share a small example on how to use this feature in a Symfony 5.1 application? That way, someone can start documenting this feature It's a huge PR, so I'm sorry if it's already shown somewhere and I just missed it.

[nicolas-grekas]

Since symfony/symfony#35924, installing amphp/http-client is all that is needed, see also symfony/symfony#36137

[nicolas-grekas]

The doc page should be updated when it tells that curl is required to leverage true async + http2: there's another option now.