ISSUE-13717: PIP-135: Include MetadataStore backend for Etcd

[sijie]

Original Issue: apache#13717

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Motivation

Since all the pieces that composed the proposal in PIP-45 were finally merged
and are currently ready for 2.10 release, it is now possible to add other
metadata backends that can be used to support a BookKeeper + Pulsar cluster.

One of the popular systems that is most commonly used in alternative of
ZooKeeper is Etcd, thus it makes sense to have this as the first non-zookeeper
implementation.

Goal

Provide a Etcd implementation for the MetadataStore API. This will allow
users to deploy Pulsar clusters using Etcd service for the metadata and it will
not require the presence of ZooKeeper.

Implementation

• Use the existing JEtcd Java client library for Etcd
• Extends the AbstractBatchedMetadataStore class, in order to reuse the
  transparent batching logic that will be shared with the ZookKeeper
  implementation.

Work in progress: apache#13225