feat(http2): implement 103 Early Hints support

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

- Add builder opt-in pattern via enable_informational() for zero-cost abstraction
- Create early_hints_pusher() API for server-side hint transmission via mpsc channel
- Add 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 test coverage: 11 integration tests, 18 unit tests, 2 doc tests
- Add complete working example with TLS, resource preloading, and performance monitoring
- Update Cargo.toml with h2 = 0.4.13 dependency requirement

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. Feature is opt-in and disabled by default for zero overhead when not used.
Clients can register callbacks to process 103 Early Hints and other informational responses.

Closes #3980, #2426

Author: Apu Islam

Cargo.toml

@@ -65,6 +65,10 @@ 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
@@ -84,7 +88,7 @@ http2 = ["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"]
@@ -201,6 +205,11 @@ 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.