Support HTTP/2 after proxy CONNECT

[rampreeth]

Motivation

When an HTTPS request uses an HTTP proxy, AHC establishes a CONNECT tunnel and then performs TLS with the target. If ALPN selects h2, the tunnel path currently leaves the HTTP/1.1 codec in place. The target then receives HTTP/1.1 bytes on a connection that negotiated HTTP/2 and closes it.

Modification

• Check the negotiated ALPN protocol after the target TLS handshake in both proxy tunnel paths.
• When ALPN selects h2, upgrade the existing channel pipeline to HTTP/2 and register the physical tunnel connection with its existing pool partition key.
• Preserve the existing HTTP/1.1 fallback and WebSocket behavior.
• Add regression tests using local HTTP and HTTPS CONNECT proxies with a TLS HTTP/2 target.
• Verify that the upgraded tunnel is registered and reused for a second request.
• Preserve the existing public tunneling method signatures for binary compatibility.

Result

HTTPS requests through HTTP and HTTPS CONNECT proxies now send HTTP/2 frames when the target negotiates h2. The regression tests verify successful HTTP/2 responses and reuse of a single physical target connection.

Testing

• ./mvnw -pl client -Dtest=BasicHttp2Test,HttpsProxyTest test — 60 tests passed.
• Full client test suite passed as part of verify; the local run then stopped at artifact signing because gpg is not installed.
• ./mvnw -pl client -DskipTests -Dgpg.skip=true verify — packaging and API compatibility checks passed.

Fixes #2241

[hyperxpro]

Not in this PR's diff, so leaving it here: this change makes HTTP/2 after CONNECT work, which makes the workaround in client/src/test/java/org/asynchttpclient/proxy/HttpsProxyTestcontainersIntegrationTest.java stale and its comment misleading. Could you remove .setHttp2Enabled(false) and the // HTTP/2 ALPN upgrade after proxy CONNECT tunnel is not yet supported comment in both testHttpProxyToHttpsTarget (lines 149-150) and testHttpsProxyToHttpsTarget (lines 172-173)? That lets those external tests exercise the new HTTP/2-through-proxy path instead of forcing HTTP/1.1, and the comment stops claiming something that is no longer true.

[rampreeth]

Done - removed .setHttp2Enabled(false) and the stale comment from both proxy-to-HTTPS Testcontainers tests. They now use the default HTTP/2-enabled configuration.

[rampreeth]

Hi @hyperxpro, just a gentle reminder when you get a chance. I've addressed the review comments and would appreciate a re-review. Thanks!

[hyperxpro]

Thanks a lot!