add a co-processor for config update, to allow customized behavior on config loading.

*Title*: add a co-processor for config update, to allow customized behavior on config loading.

*Description*:
At the present, the controlplane/config loading oof Envoy is relatively "closed" compared to the highly "configurable" dataplane portion. It's really hard to debug if the Xds status gets into a skewed status.

Similar to HBase coprocessor, we could have a [list of] coprocessors registered and function on the onConfigUpdate grpc_mux_impl or at every XDS callbacks's onConfigUpdate, to offer possibly better customized (and very likely not general enough to be upstreamed) behaviors for Envoy config loading path:
* they could either be in-place mutating the received config,
* or even just receiving a const reference of the received resources. (observer/immutable)