xds: Plumb locality in xds_cluster_impl and weighted_target (1.64.x backport)

[ejona86]

As part of gRFC A78:

To support the locality label in the WRR metrics, we will extend the
weighted_target LB policy (see A28) to define a resolver attribute
that indicates the name of its child. This attribute will be passed
down to each of its children with the appropriate value, so that any
LB policy that sits underneath the weighted_target policy will be
able to use it.

xds_cluster_impl is involved because it uses the child names in the AddressFilter, which must match the names used by weighted_target. Instead of using Locality.toString() in multiple policies and assuming the policies agree, we now have xds_cluster_impl decide the locality's name and pass it down explicitly. This allows us to change the name format to match gRFC A78:

If locality information is available, the value of this label will be
of the form {region="${REGION}", zone="${ZONE}", sub_zone="${SUB_ZONE}"}, where ${REGION}, ${ZONE}, and
${SUB_ZONE} are replaced with the actual values. If no locality
information is available, the label will be set to the empty string.

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CC @DNVindhya, @temawi. The API exposed to WRR will be WeightedTargetLoadBalancer.CHILD_NAME

Backport of #11133