Cached ssl contexts.

[T-256]

(Version 1.0 proposal)

Now we have ssl_context=... for ssl context configuration for each client. When not used, by default we are create new isolated httpx.SSLContext per client instantiate. it's expensive and brings memory leak for high scale and multiple client uses. We want single global ssl context to use for client's default uses (no ssl_context=... passed).

clients in httpx can be configured to be compatible with HTTP/1.1 or HTTP2 versions. TLS streams have ALPN (Application-Layer Protocol Negotiation) option in client hello packet to say to server what protocol could be used without extra roundtrip/connection upgrades. httpx should keep supported with ALPN standard. So, it should auto add ALPNs from passed http versions in Client instantiate.

In #3340 prototype I added cached layer to httpx.SSLContext to support default ssl context. It uses class method which uses cached (or one-time generating if not available) ssl contexts for different http versions configurations.
I tried to keep related constructor to httpx.SSLContext, but I'm open to extract it to global context which then would be:

# httpx/utils.py
from ._config import SSLContext

_ssl_contexts: dict[tuple[bool, bool], "SSLContext"] = {}


def get_cached_ssl_context(http1: bool = True, http2: bool = False) -> SSLContext:
    """
    If there is SSLContext for provided http versions return that, otherwise create new.
    """
    context = _ssl_contexts.get((http1, http2))
    if context is not None:
        return context

    context = SSLContext()
    if ssl.HAS_ALPN:
        alpn_idents = []
        if http1:
            alpn_idents.append("http/1.1")
        if http2:
            alpn_idents.append("h2")
        context.set_alpn_protocols(alpn_idents)

    _ssl_contexts[(http1, http2)] = context
    return context