Allow the cURL transport to use native HTTP protocol negotiation

Is your feature request related to a problem?

Requests defaults protocol_version to HTTP/1.1 and the cURL transport therefore explicitly sets:

CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1

This prevents libcurl from using its native HTTP protocol negotiation, even when both libcurl and the server support HTTP/2 or newer protocols.

This is related to #381 and PR #918, but is intentionally narrower. Instead of adding explicit HTTP/2 / HTTP/3 support, this issue is about allowing capable transports to choose the protocol themselves.

Describe the solution you'd like

Add a way to request transport-native protocol negotiation, for example with null or "auto".

Conceptually:

1.0  -> force HTTP/1.0
1.1  -> force HTTP/1.1
auto -> let the transport decide

For cURL, auto would mean not setting CURLOPT_HTTP_VERSION and letting libcurl negotiate the protocol.

For fsockopen, Requests could continue using the HTTP/1.x version it supports.

Describe alternatives you've considered

PR #918 adds explicit 2.0 and 3.0 protocol versions, but that also introduces questions around transport capabilities, HTTP/3 and testing infrastructure.

Another workaround is changing the cURL handle through hooks, but that requires downstream applications to know that Requests otherwise forces HTTP/1.1.

Additional context

This came up while investigating HTTP/2 support in WP-CLI.

A native-negotiation mode could be tested using CURLINFO_HTTP_VERSION against an HTTP/2-capable test server, while preserving HTTP/1.1 fallback when HTTP/2 is unavailable.

The main design question is whether protocol_version should always dictate the protocol for every transport, or whether protocol selection can remain a transport capability when the caller does not explicitly request a version.

Start by tracing protocol_version handling in the cURL and fsockopen transports, including how CURLOPT_HTTP_VERSION is set. Resolve whether an auto or null mode should leave protocol selection to cURL while preserving HTTP/1.x behavior for fsockopen. Validate the result with CURLINFO_HTTP_VERSION against an HTTP/2-capable test server and confirm HTTP/1.1 fallback.