Introduce Broadcast API

[sejongk]

What this PR does / why we need it:
This pull request introduces the new "broadcast" feature, allowing for the sharing of general events.

• Add Document.Broadcast API
  • Update protocol: Add broadcast-related types
  • SDK: Implement Document.broadcast(event)
  • Server: Implement Broadcast API
• SDK: Implement Document.Subscribe

Which issue(s) this PR fixes:

Address #628

Special notes for your reviewer:
The following are the current implementations that need to be discussed.
I added test cases for the below specs.

• Clients who don't subscribe to an event type can broadcast for that event type.
• The payload in a broadcast event should be serializable to JSON.
• Clients can keep watching the document although an error occurs during the handling of a broadcast event.

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: 165 lines in your changes are missing coverage. Please review.

Comparison is base (940941a) 49.47% compared to head (bde74a4) 48.86%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #631      +/-   ##
==========================================
- Coverage   49.47%   48.86%   -0.61%     
==========================================
  Files          69       69              
  Lines        9951    10082     +131     
==========================================
+ Hits         4923     4927       +4     
- Misses       4512     4635     +123     
- Partials      516      520       +4     

Files	Coverage Δ
api/types/auth_webhook.go	34.00% <100.00%> (+1.34%)	⬆️
pkg/document/time/actor_id.go	55.76% <ø> (ø)
api/converter/from_pb.go	41.34% <0.00%> (-0.75%)	⬇️
api/converter/to_pb.go	52.48% <0.00%> (-0.27%)	⬇️
server/backend/sync/memory/coordinator.go	31.25% <0.00%> (-1.01%)	⬇️
pkg/document/document.go	38.12% <25.92%> (-2.91%)	⬇️
server/rpc/yorkie_server.go	44.98% <0.00%> (-5.54%)	⬇️
client/client.go	12.35% <0.00%> (-1.23%)	⬇️

... and 5 files with indirect coverage changes

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[sejongk]

@hackerwins
There are some points to note regarding this implementation:

1. DocumentBroadcast is one of the DocEvent types, meaning that broadcast events are delivered as DocEvent objects through the watch stream.
2. Given the frequency of broadcasts and the complexity of the implementation, it appears to be a good practice to always send broadcasts to all clients, allowing clients to filter them out locally. If necessary, we can consider implementing server-side event filtering at a later stage.
3. The PubSub module can remain mostly unchanged since we won't be implementing server-side event filtering as mentioned above.

[hackerwins]

Thanks for your contribution.

It would be great to refine SubscribeBroadcastEvent along with integrating the Attach and Watch interfaces(#584).