feat(tonic-xds): add transport-generic build_transport_channel

[LYZJU2019]

What

Adds XdsChannelBuilder::build_transport_channel, a transport-generic build path that wires the xDS routing → retry → load-balancing stack over a caller-provided MakeConnector and returns a type-erased, cloneable Send + Sync tower::Service. The existing gRPC channel becomes the special case of this shared stack.

This is an additive, non-breaking change — no existing public API changes behavior, and the gRPC connect path and all gRPC tests are untouched.

Why

Follow-up to the xDS transport-generalization work (#2750, #2751, #2776). Downstream users that speak a non-gRPC transport (e.g. plain HTTP) need to drive the same xDS routing/retry/LB machinery with their own per-endpoint connector, without reimplementing the stack.

Details

• Shared core: extracted a private build_stack that both the gRPC and generic paths call; the gRPC path (build_from_cache) is now a thin caller. Also extracted load_bootstrap / build_xds_runtime helpers. The cert-provider registry stays gRPC-only (built before the bootstrap node is consumed, so the generic path never pays for it).
• Body flow: the retry engine converts Request<B> → Request<SharedBody<B>> so a request can be cloned for replay; a post_retry_map bridges back to the body the per-endpoint service expects. gRPC rewraps into a tonic body (unchanged); the generic path's connector accepts SharedBody<B> directly (identity map).
• Public retry seam: made RetryClassifier, RetryConfig, RetryBackoffConfig, and is_retryable_connection_error public so non-gRPC transports can plug their own retry logic. Retry behavior is threaded into build_transport_channel as a classifier + RetryConfig; the internal RetryPolicy (and its tower::retry::Policy impl) stays private to keep the public surface minimal.
• check-external-types: allowlisted shared_http_body — the caller's connector accepts a SharedBody-wrapped body.

Testing

• New e2e test (xds_channel_e2e_transport_generic_routes_to_backend) drives the generic path end-to-end through the mock control plane, using a custom RetryClassifier + a SharedBody-accepting connector written entirely against the public API.
• Build/test/doc with -D warnings (400 tests + 4 doctests), clippy, rustfmt, and check-external-types --all-features all pass.

[LYZJU2019]

Superseded by #2786, which reworks the retry configuration to be control-plane-driven: instead of a static RetryConfig parameter, build_transport_channel now takes a Fn(&RouteConfigMetadata) -> Option<RetryConfig> closure and a background task hot-swaps the config from RouteConfiguration updates.