Fix RLS config child lb config validation

[dapengzhang0]

As per the spec go/grpc-rls-lb-policy-design

At config-validation time, we must construct and validate the config for the child policy, but we don't yet know the target name to use for it. The config for the child policy will be constructed by starting with the static config from the childPolicy field, adding the field name specified in childPolicyConfigTargetFieldName, and setting the value of that field to some dummy string.