Support username change (WIP)

[doyoonear]

What this PR does / why we need it?

• Support username change for identifying

Any background context you want to provide?

What are the relevant tickets?

Enhances #129

Checklist

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

[CLAassistant]

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[hackerwins]

Related with https://github.com/yorkie-team/yorkie-js-sdk/issues/362.

[blurfx]

@doyoonear are you still working on it? it seems staled.

cc. @hackerwins

[hackerwins]

@doyoonear Thanks for your contribution.

This feature looks useful but has not been updated on for a long time. I'll close this for now. Please let me know if you have any plans to work on this.