Proto commits in etcdv3/etcd-client

These 7 commits are when the Protocol Buffers files have changed:

Commit:e533eb5
Author:Dimitris Apostolou
Committer:GitHub

Fix typos (#51)

The documentation is generated from this commit.

Commit:2e9dd5d
Author:ivorhuang-hjd
Committer:GitHub

Add maintenance APIs . (#6) Add maintian APIs. Co-authored-by: huangjingdong <huangjingdong@sics.ac.cn> Co-authored-by: David Li <davidli2010@foxmail.com>

Commit:da9a4cc
Author:David Li

Watch operation.

Commit:b345173
Author:David Li

KV: transaction operation.

Commit:a8a62b7
Author:David Li

KV: get operation.

Commit:d632559
Author:David Li

Compile proto files.

Commit:da953c4
Author:David Li

Add original etcd API proto files copied from etcd release-3.4@cdb2dc11b.