gRPC-xDS: xDS Federation (A47)

I don't have the rights to create a sub issue of https://github.com/grpc/grpc-rust/issues/2753

Feature Request

Crates

The implementation is scoped to xds-client/tonic-xds crate

Motivation

xds-client at the moment does not support multiple authority

Proposal

Add support for https://github.com/grpc/proposal/blob/master/A47-xds-federation.md

Rough plan

[ ] Add canonical xdstp resource-name parsing [ ] Add authority configuration types [ ] Refactor the single ADS worker into actor plus server tasks [ ] Add authority-aware resource routing and caching

Alternatives

Open questions (more will come as I make progress)

I'm missing some information on current crates interactions and evolution.

• grpc-xds would eventually need use the boostrap config struct
• Deadlock issue https://github.com/grpc/grpc-rust/issues/2778#issuecomment-5184644089 might require some changes

Proposed update

Blocking behavior

• The client worker historically owned one ADS stream, so all subscriptions were sent to the top-level server. Per-authority routing requires independent stream state, reconnect, nonce/version, backoff, and ACK/NACK ordering.
• Server identity cannot be the URI alone. Stream deduplication must include URI, credentials, and server features.
• Resource/cache state keyed only by a bare name can collide across authorities. The full canonical resource name must be retained through LDS, RDS, CDS, and EDS.
• Bootstrap authorities and listener templates cannot be enabled until watches are routed to the corresponding authority server; partial support could send resources to the wrong control plane.

Missing features

• xTP1 xdstp:// resource-name parsing and canonicalization.
• Authority extraction with exactly-once percent decoding.
• Context-parameter canonicalization.
• Validation that the embedded xTP1 resource type matches the watched type.
• gRFC A47 bootstrap authorities and client listener resource-name templates.
• Per-authority xDS server selection and stream sharing for identical server configurations.
• gRFC A71 priority fallback and recovery per authority.
• Per-server channel credentials and server-feature handling.
• Authority-safe cascading subscriptions and cache keys through LDS/RDS/CDS/EDS.

Issues found

• Connection construction, stream creation, and stream write failures did not always reach the worker, preventing fallback and disconnected metrics.
• Wildcard subscriptions could be replaced by named subscriptions on later requests or ACKs.
• An invalid replacement could be marked NACKedButCached and then incorrectly overwritten as deleted.
• ACK/NACK tasks could complete out of order on the same ADS stream.
• Per-resource validation errors did not retain ProcessingDone, allowing a NACK before watcher processing completed.
• A dirty subscription marker could be lost when the command channel remained full.
• Stale dirty and ACK state could remain after a stream was removed.
• URI authorities could be decoded twice, causing distinct encoded authorities to collide.
• Decoded authorities were not always percent-encoded when rebuilding target URIs or listener templates.
• Malformed xdstp: names could fall back to legacy-name handling.
• xdstp names with the wrong embedded resource type could be subscribed to or accepted.
• Mutable or debug-derived server-extension identity could invalidate hash keys or expose credential data.