Add Client sync mode

[chacha912]

What this PR does / why we need it?

• Implement Client Sync mode
  • In pushOnly mode,
    • Set pushOnly flag in the PushPull API.
    • Don't apply remote changes.

Any background context you want to provide?

What are the relevant tickets?

Related yorkie-team/yorkie#500, yorkie-team/yorkie-ios-sdk#65

Checklist

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

[codecov]

Codecov Report

Merging #495 (3ec225a) into main (6e2c501) will increase coverage by 0.12%.
The diff coverage is 93.49%.

@@            Coverage Diff             @@
##             main     #495      +/-   ##
==========================================
+ Coverage   90.04%   90.17%   +0.12%     
==========================================
  Files          69       70       +1     
  Lines        5989     6148     +159     
  Branches      569      578       +9     
==========================================
+ Hits         5393     5544     +151     
- Misses        388      392       +4     
- Partials      208      212       +4     

Impacted Files	Coverage Δ
src/yorkie.ts	100.00% <ø> (ø)
src/client/client.ts	73.53% <64.51%> (-0.81%)	⬇️
src/client/attachment.ts	77.77% <100.00%> (+1.03%)	⬆️
src/document/document.ts	73.29% <100.00%> (+0.50%)	⬆️
test/integration/client_test.ts	99.78% <100.00%> (+0.08%)	⬆️

... and 2 files with indirect coverage changes

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