Add context.Context Support and Improve Context Handling in Stream Methods

[molon]

• All rest methods now provide proper context.Context support
• Clarified behavior of context parameters in streaming methods:
  • In Connect method, ctx only controls the initial connection process, not the full stream lifecycle
  • To terminate a stream created by Connect, use the returned terminate function or the Terminate method
  • In StreamTradeUpdatesInBackground, ctx only controls the initial connection, stream continues running independently afterward
  • To stop a stream started by StreamTradeUpdatesInBackground, use the returned terminate function
• Replaced time.Sleep with ctxtime.Sleep to ensure timely cancellation on context expiration
• Removed default timeout from handleSubChange, now uniformly controlled by ctx
• Added buffer to subChangeRequest.result to prevent deadlocks caused by handleSubChange error returns in edge cases

These changes improve context handling throughout the API, making resource management more predictable and preventing potential deadlocks.

[gnvk]

@molon This is a giant backward incompatible change. There are two ways to get this merged:

1. Once reviewed and approved merge it to the v4 branch. There are a couple more issues labeled with v4. Once we do them all, we can consider releasing a new major version of the SDK. One of the benefits of this approach is that we would encourage our users to migrate to the context-aware methods.
2. Instead of adding context to all current client methods, we could duplicate them all. E.g. client.GetAccount() to client.GetAccountCtx(context.Context). This way we could keep the major version, however it's an even bigger change. Moreover, it will possibly increase maintenance cost in the future.

[molon]

@gnvk

Sorry for the late reply. I prefer the first way.

Additionally, I'm debating one issue.
Since I changed the behavior of the ctx parameter in streaming methods, even if we go with the first way, should we rename streaming methods to make users more aware of this backward incompatibility?