feat(tonic-xds): implement xds resources and route matchers

[YutaoMa]

Motivation

Ref: #2444

xDS Resource Types and Route Matching

xds-client (PR #2478 ) handles the ADS protocol but knows nothing about gRPC. tonic-xds has its Tower service stack scaffolded but lacks the middle layer bridging xDS updates to routing decisions. This PR builds that foundation: validated resource types and a route matching engine that the upcoming resource manager will use to drive the LDS→RDS→CDS→EDS cascade and feed the existing Tower services.

This is the 1st PR of a series to complete tonic-xds. The next steps include:

• xDS Resource cache
• Endpoint management: EDS diffing into tower::discover::Change streams
• Resource manager: cascade task managing LDS→RDS→CDS→EDS watches, implementing XdsRouter + XdsClusterDiscovery
• Bootstrap config: JSON parsing from env vars → xds_client::ClientConfig
• Builder wiring: wire full stack, E2E test
• Advanced routing (gRFC A28+A63): Weighted cluster selection
• Least request LB (gRFC A48)

Solution

Implements the xds_client::Resource trait for the four gRPC xDS resource types and adds a route compilation/matching engine, bridging xds-client watches to validated, efficient routing structures:

• Resource types xds/resource/: ListenerResource (LDS), RouteConfigResource (RDS), ClusterResource (CDS), EndpointsResource (EDS) — each implements xds_client::Resource with two-phase decode (deserialize + validate).
• Route matching xds/route/: CompiledRouteConfig pre-compiles a RouteConfigResource at RDS update time (regex compilation, domain indexing), allows for allocation-free per-request matching.
• Refactors: RouteInput/RouteDecision moved to client/route (since it's used for Tower layer specifically, rather than the xDS subscription side of route logic this PR deals with).