feat: add sync mode for subscription events

[hgiasac]

close #113

Add a new syncMode option to allow subscriptions to execute messages in sequence (without goroutine)

client.WithSyncMode(true)

[andrexus]

I would prefer to have the "sync mode" as the default because, in my opinion, the GraphQL protocol heavily relies on the sequence of messages. I've spent a couple of hours debugging the root cause of the problem, and I think some people may not read the entire documentation but rather create a new issue.
With an extra option, users could enable the "async mode" for improved performance.
As I said, this is just my opinion, and you can decide what defaults the library should have. I am also quite happy with your current solution. Thank you.

[hgiasac]

I haven't tested in-depth about extremely high race condition use cases. However, there would be some locks on the read-message loop. For example, if the execution is too long, other messages will be locked or discarded from the WebSocket client.
So we should keep the async mode to avoid breaking changes for other users. It would be great if you could test and prove the sync mode works without issue. Thanks

[andrexus]

I have heavily tested this version in my project, which has numerous subscriptions, and I have not encountered any issues. I think it can be merged.