feat(tonic-xds): add A39 HTTP filter registry and router filter

[mingley]

Summary

This implements gRFC A39.

What A39 does. The control plane can attach extra per-request rules to a gRPC client: delay this call, abort that one, and so on. Each rule is a labeled blob (name + proto type + bytes). The client keeps a dictionary of types it understands. Unknown required types reject the config; unknown optional types are ignored. Every filter list must end with the router filter, which is just a marker meaning "stop, now route."

What this PR does. It adds that dictionary to tonic-xds and teaches it to read one blob.

• The only registered type is envoy.extensions.filters.http.router.v3.Router. We accept it and ignore its fields.
• Two wrappers appear on the wire. FilterConfig is a box around the real config and can say "optional." TypedStruct is a box that only names the inner type. We unwrap both before looking in the dictionary.
• Empty names, missing config, ConfigDiscovery, and disabled=true are rejected. Router overrides are rejected (the router has no override message).

It does not walk a listener's filter list, merge route overrides, or run anything on the request path.

What comes next.

1. Parse HttpConnectionManager.http_filters on the Listener (empty list = implicit router).
2. Parse typed_per_filter_config on virtual hosts, routes, and weighted clusters.
3. Merge those layers (weighted cluster > route > virtual host > listener).
4. Attach the merged list to the routing decision so later filters (fault injection, RBAC) can run.

Part of #2444.

Testing Done

• cargo fmt -p tonic-xds -- --check
• cargo clippy -p tonic-xds --all-targets --all-features -- -D warnings
• cargo test -p tonic-xds --lib http_filter — 23 passed (4dcd1cf3)