Update protocol to support multi-user undo and redo on Text.edit

[hyemmie]

What this PR does / why we need it: A new operation, EditReverse, is proposed to support multi-user undo and redo functionality in text.edit. (yorkie-team/yorkie-js-sdk#635)
To support this, the server also needs to modify the protocol buffer. This PR defines the new EditReverse operation and modifies the protocol buffer to allow replicas to send and receive EditReverse operations.

Which issue(s) this PR fixes:

It fixs #49 partially.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 56 lines in your changes missing coverage. Please review.

Project coverage is 49.19%. Comparing base (940941a) to head (57e196e).
Report is 340 commits behind head on main.

Files with missing lines	Patch %	Lines
api/converter/from_pb.go	0.00%	34 Missing ⚠️
api/converter/to_pb.go	0.00%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
- Coverage   49.47%   49.19%   -0.28%     
==========================================
  Files          69       69              
  Lines        9951    10007      +56     
==========================================
  Hits         4923     4923              
- Misses       4512     4568      +56     
  Partials      516      516              

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