Add support of managing the raft cluster via the client

[git-hulk]

❯ _build/kvctl raft list peers

|---------|-----------------------|-----------|
| NODE ID |     NODE ADDRESS      | IS LEADER |
|---------|-----------------------|-----------|
|       1 | http://127.0.0.1:6001 | YES       |
|---------|-----------------------|-----------|

❯ _build/kvctl raft add peer 2 http://127.0.0.1:6002
Add node '2' with address 'http://127.0.0.1:6002' successfully

❯ _build/kvctl -H "http://127.0.0.1:9380" raft remove peer 2
Remove node '2' successfully

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 106 lines in your changes missing coverage. Please review.

Project coverage is 46.16%. Comparing base (6c56470) to head (1a87e70).
Report is 34 commits behind head on unstable.

Files with missing lines	Patch %	Lines
cmd/client/command/raft.go	0.00%	105 Missing ⚠️
cmd/client/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #258      +/-   ##
============================================
+ Coverage     43.38%   46.16%   +2.77%     
============================================
  Files            37       44       +7     
  Lines          2971     4038    +1067     
============================================
+ Hits           1289     1864     +575     
- Misses         1544     1980     +436     
- Partials        138      194      +56     

Flag	Coverage Δ
unittests	46.16% <0.00%> (+2.77%)	⬆️

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