xds/resolver: pass route's auto_host_rewrite to LB picker (gRFC A81) (#8740)

xds/resolver: pass route's auto_host_rewrite to LB picker (gRFC A81) (#8740)

This PR implements the ConfigSelector changes required for [gRFC
A81](https://github.com/grpc/proposal/blob/master/A81-xds-authority-rewriting.md).

It ensures that the `auto_host_rewrite` field from the xDS Route
Configuration is correctly propagated through the resolver and made
available to the Load Balancer picker via the RPC context.

### Key Changes:

* Pass the `AutoHostRewrite` field value from `Route` struct via RPC
context.
* Add helper functions for `AutoHostRewrite` in
`internal/xds/balancer/cluserimpl/picker.go`.
* Update `ConfigSelector.SelectConfig` to pass the `AutoHostRewrite`
boolean in RPC context.

RELEASE NOTES: None