feat(http2): implement 103 Early Hints support

Add complete HTTP/2 103 Early Hints implementation with client and server support:

- Add InformationalSender extension for server-side hint transmission via mpsc channel
- Create InformationalCallback system for client-side informational response handling
- Extend HTTP/2 client builder with informational_responses() configuration method
- Implement informational response polling in h2 client task with callback invocation
- Add server-side informational response forwarding using h2's send_informational API
- Include extensive integration tests covering multiple scenarios and edge cases
- Add `enable_informational` config field (defaults to false) and conditional channel creation per request
- Add complete working example with TLS, resource preloading, and performance monitoring
- Update Cargo.toml with local h2 dependency and example build configuration

The implementation enables servers to send resource preload hints before final responses,
allowing browsers to start downloading critical resources early and improve page load
performance. Clients can register callbacks to process 103 Early Hints and other
informational responses.

Closes #3980, #2426

Author: Apu Islam

Cargo.toml

@@ -67,6 +67,10 @@ tokio = { version = "1", features = [
 ] }
 tokio-test = "0.4"
 tokio-util = "0.7.10"
+# Additional dependencies for HTTP/2 Early Hints example
+rcgen = "0.12"
+tokio-rustls = "0.25"
+rustls-pemfile = "2.0"
 
 [features]
 # Nothing by default
@@ -86,7 +90,7 @@ http2 = ["dep:atomic-waker", "dep:futures-channel", "dep:futures-core", "dep:h2"
 
 # Client/Server
 client = ["dep:want", "dep:pin-project-lite", "dep:smallvec"]
-server = ["dep:httpdate", "dep:pin-project-lite", "dep:smallvec"]
+server = ["dep:httpdate", "dep:pin-project-lite", "dep:smallvec", "dep:futures-util"]
 
 # C-API support (currently unstable (no semver))
 ffi = ["dep:http-body-util", "dep:futures-util"]
@@ -305,6 +309,11 @@ name = "web_api"
 path = "examples/web_api.rs"
 required-features = ["full"]
 
+[[example]]
+name = "http2_early_hints"
+path = "examples/http2_early_hints.rs"
+required-features = ["full"]
+
 
 [[bench]]
 name = "body"

examples/README.md

@@ -38,6 +38,8 @@ futures-util = { version = "0.3", default-features = false }
 
 * [`echo`](echo.rs) - An echo server that copies POST request's content to the response content.
 
+* [`http2_early_hints`](http2_early_hints.rs) - An HTTP/2 server that sends 103 Early Hints.
+
 ## Going Further
 
 * [`gateway`](gateway.rs) - A server gateway (reverse proxy) that proxies to the `hello` service above.