xds-client: add canonical xdstp resource-name parsing

[W4lspirit]

Motivation

gRFC A47 identifies federated xDS resources with xdstp:// URIs. These names need stable equality and hashing across percent-encoding and context-parameter order, while their opaque paths must not be changed.

I'm not using the url crate as it will normalizing the path:

• https://url.spec.whatwg.org/
• https://docs.rs/url/latest/url/

This parsing foundation is kept separate from authority routing so it can be reviewed and merged without changing the behavior of existing non-federation clients.

What changed

• Add ResourceName and AuthorityKey for legacy and xdstp resource names.
• Structurally split xDS URIs without normalizing .
• Decode URI components exactly once and retain a canonical wire representation.
• Canonically order context parameters and normalize percent encoding.
• Distinguish legacy, empty-authority, and named-authority resource keys.
• Reject malformed names using the reserved xdstp: scheme.
• Add focused parsing, identity, encoding, and round-trip tests.

Compatibility

This PR does not connect the parser to watch routing, bootstrap authorities, or server selection. Legacy resource names round-trip byte-for-byte, and the existing single-server path is unchanged.

Validation

• cargo test -p xds-client
• cargo clippy -p xds-client --all-targets -- -D warnings
• Behavior compared with the corresponding gRPC-Go, gRPC-Java, and gRPC C++ resource-name implementations.

Part of #2795.