Improve Peer Awareness's metadata to be updatable

[hackerwins]

What would you like to be added:

Realtime collaborative editors usually need to propagate volatile data, such as cursor position to other peers. This type of data needs to be propagated, but not stored.

For now, we implemented Peer Awareness to propagate the metadata of the client, but updates are not possible. We can cover this scenario by improving the Peer Awareness's metadata so that it can be updated.

Why is this needed:

The cost of storing data is expensive, so it is economical to propagate this type of data but not store it.

[dc7303]

@hackerwins could I try this issue?

[hackerwins]

@dc7303 Yes. of course. This issue is more complicated than other issues, so writing a design document together would be nice.

[hackerwins]

Metadata for the purpose of sharing the status of peers who are watching the document together is stored in the SubscriptionMap as non-persistent data.

Since the client receives Peer's changes using Watch stream, it would be good to implement updating metadata using this stream as well. However, grpc-web does not support bi-directional streams now.

Therefore, we plan to temporarily update the metadata using Unary and remove it when the bidirectional stream is supported in grpc-web in the future.