Fix: proxy http2 prior to knowledge support

[p1-ra]

Hi,

This PR is related to #1624 and required the following PR at httpcore side: encode/httpcore#342
It fix the HTTP2 prior of knowledge support for HTTP proxies.

• 5a34fdd propagate client http1 support to the configured proxies
• 9825d69 introduce new configurations on the Proxy object to override the client http capabilities: http1_override, http2_override

proxies = {
    # Route all traffic through a proxy by default, inherit http1,http2 capabilities from client
    "all://": "http://localhost:8030",
    # But don't use proxies for HTTPS requests to "domain.io"...
    "https://domain.io": None,
    # And use another proxy for requests to "example.com" and its subdomains, only support http1
    "all://*example.com": httpx.Proxy(url="http://localhost:8031", http1_override=True, http2_override=False),
    # And yet another proxy if HTTP is used,
    # and the "internal" subdomain on port 5550 is requested, only support http2 -- prior to knowlegdge
    "http://internal.example.com:5550": httpx.Proxy(url="http://localhost:8032", http1_override=False, http2_override=True),
}

BR,

• Initially raised as discussion #None

[p1-ra]

@tomchristie @florimondmanca any input/feedbacks regarding this?

I've noticed that encode/httpcore#342 has been closed without comment.

Any reasons to not be able to use HTTP/2 prior to knowledge with proxy?

Does the issue have been resolved elsewhere?

Thanks in advance

[tomchristie]

Resolved via #2009 and encode/httpcore#468