Provide RS coproc ability to attach WAL extended attributes to mutations at replication sink

HBase provides coproc ability to enhance WALKey attributes (a.k.a. WAL annotations) in order for the replication sink cluster to build required metadata with the mutations. The endpoint is preWALAppend(). This ability was provided by HBASE-22622. The map of extended attributes is optional and hence not directly used by hbase internally. 

For any hbase downstreamers to build CDC (Change Data Capture) like functionality, it might required additional metadata in addition to the ones being used by hbase already (replication scope, list of cluster ids, seq id, table name, region id etc). For instance, Phoenix uses many additional attributes like tenant id, schema name, table type etc.
We already have this extended map of attributes available in WAL protobuf, to provide us the capability to (de)serialize it. While creating new ReplicateWALEntryRequest from the list of WAL entires, we are able to serialize the additional attributes. Similarly, at the replication sink side, the deserialized WALEntry has the extended attributes available.

At the sink cluster, we should be able to attach the deserialized extended attributes to the newly generated mutations so that the peer cluster can utilize the mutation attributes to re-build required metadata.