[T3 Connect] Negotiate HTTP/2 for procedure HTTPS requests

[bradleyshep]

Part of the work to move T3 Code's T3 Connect relay onto SpacetimeDB.

Description of Changes

This enables reqwest's native-tls-alpn feature, so the native-tls connector offers ALPN h2. Without it, every outbound ctx.http.fetch over HTTPS was pinned to HTTP/1.1, and HTTP/2-only APIs could not be reached from modules. One example is Apple's APNs, which modules need for iOS push notifications.

Split out of #6005 / #6006.

API and ABI breaking changes

None.

Rollback safety impact

n/a

Expected complexity level and risk

1. It's a one-line feature flag. Servers that only speak HTTP/1.1 keep negotiating HTTP/1.1.

Worth a reviewer's eye: the flag is on the workspace reqwest, and release builds compile the CLI, updater and standalone server together, so the CLI and updater also start offering HTTP/2 over TLS. Scoping it to spacetimedb-core wouldn't change that, because Cargo unifies features across one build. Maincloud and GitHub select HTTP/2 when offered; the S3 bucket for client binaries stays on HTTP/1.1. spacetime server ping maincloud works with a build of this branch; publish, logs and spacetime version upgrade over HTTP/2 weren't tested.

Testing

• New test http_request_offers_h2_over_tls (behind allow_loopback_http_for_tests, like the decompression tests beside it): a procedure HTTPS request made through InstanceEnv::http_request offers h2 via ALPN in its TLS ClientHello. Without this change it offers no ALPN protocols, so an HTTP/2-only server such as APNs has nothing to select.
• The test fails with native-tls-alpn removed
• Not covered: a full HTTP/2 round trip. The procedure client only trusts the OS certificate store, so a loopback server can't complete the handshake without a production seam (e.g. a shared client builder a test could add a root certificate to).
• Note: CI's cargo test --all doesn't enable allow_loopback_http_for_tests, so this test doesn't run in CI on its own; [T3 Connect] Reuse one HTTP client per replica for procedure requests #6023 adds a focused CI step that runs the loopback HTTP tests, including this one once both land.
• CI build

[alexscomm]

I need this fix for iOS push notifications.