Add RealtimeSyncOff and refactor interface of SyncMode

[chacha912]

What this PR does / why we need it?

The SyncOff mode will allow users to maintain the watch stream functionality while disabling synchronization.

Does this PR introduce a user-facing change?:

- 1️⃣ With the addition of the `RealtimeSyncOff` mode, the `SyncMode` has been simplified into four options. Usage is as follows:

// as-is
await client.pause(doc); // manual sync
await client.resume(doc); // realtime sync(pushpull)
client.pauseRemoteChanges(doc); // pushonly 
client.resumeRemoteChanges(doc); // pushpull 

// to-be
await client.changeSyncMode(doc, SyncMode.Manual);
await client.changeSyncMode(doc, SyncMode.Realtime); // Same as PushPull mode
await client.changeSyncMode(doc, SyncMode.RealtimePushOnly); 
await client.changeSyncMode(doc, SyncMode.RealtimeSyncOff); // Added RealtimeSyncOff mode 

- 2️⃣ In manual sync mode, the `sync()` function can now only be executed in pushpull mode.

await client.sync(doc, SyncMode.PushOnly) // Deprecated: Only pushpull synchronization is supported now.

- 3️⃣ The option to specify manual sync during attachment has been updated.

// as-is 
await client.attach(doc, { isRealtimeSync: false });

// to-be
await client.attach(doc, { syncMode: SyncMode.Manual });

Any background context you want to provide?

• I'd like to discuss the user interface

1. With the addition of SyncOff mode, the existing functions (pauseRemoteChanges, resumeRemoteChanges) have been unified under changeRealtimeSyncMode.

// as-is
client.pauseRemoteChanges(doc); // pushonly 
client.resumeRemoteChanges(doc); // pushpull 

// to-be
client.changeRealtimeSyncMode(doc, SyncMode.PushPull); // pushpull
client.changeRealtimeSyncMode(doc, SyncMode.PushOnly); // pushonly
client.changeRealtimeSyncMode(doc, SyncMode.SyncOff); // syncoff

2. The changeRealtimeSyncMode is currently designated for use only in realtime sync. Therefore, if you want to resume from manual mode to pushonly mode, you need to call resume() and then change the sync mode. By default, resume() operates in push-pull mode, but would it be necessary to provide an option to set the sync mode during the resume process?

// manual -> realtime(pushonly)
// as-is
await client.pause(doc); // manual sync
await client.resume(doc); // realtime sync
client.changeRealtimeSyncMode(doc, SyncMode.PushOnly);

// provide option
await client.pause(doc); // manual sync
await client.resume(doc, {mode: SyncMode.PushOnly}); // realtime sync

• The quill-two-clients example is based on the code from Introduce mark operation to Text.setStyle #647.

What are the relevant tickets?

Fixes #770

Checklist

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

[hackerwins]

Thanks for your contribution.

From my understanding, the current structure of the SDK's SyncMode is as follows:

- Realtime: With EventStream
  - PushPull
  - PushOnly
  - SyncOff
- Manual: Without EventStream
  - PushPull
  - PushOnly
  - SyncOff

Even in Manual mode, users can set PushOnly or SyncOff, which may confuse them. How about refactoring SyncMode to have a flat structure for easier understanding?

Proposed Sync Mode options:

- SyncMode.Manual
- SyncMode.Realtime(same with `PushPull`)
- SyncMode.RealtimePushOnly
- SyncMode.RealtimeSyncOff

I think we need to keep Promise in the return and operate asynchronously only when changing from Manual to Realtime.

Perhaps the APIs will also change as shown below:

client.changeSyncMode(doc, SyncMode.Manual); // prev: await client.pause();
client.changeSyncMode(doc, SyncMode.Realtime); // prev: await client.resume();
client.changeSyncMode(doc, SyncMode.RealtimePushOnly); // prev: client.pauseRemoteChanges(doc);
client.changeSyncMode(doc, SyncMode.RealtimeSyncOff); // prev: client.resumeRemoteChanges(doc);

[chacha912]

@hackerwins Thank you for the suggestion. I agree that refactoring SyncMode into a flat structure is a good idea. So, if I understand correctly, in manual mode, only push-pull synchronization is possible? Previously, users could perform push-only synchronization even in manual mode using the sync(doc, syncMode) function. In that case, I'll also remove the syncMode option from the sync function.

[chacha912]

@hackerwins I've implemented your suggestions. Additionally, would it be better to change the isRealtimeSync option to syncMode in the attach function as well?

[hackerwins]

@chacha912 That's a good idea. I think it will be more convenient to use.

[hackerwins]

LGTM. 👍

Thank you for handling my requests during the review.
Please update the document too.
https://yorkie.dev/docs/js-sdk