Add Multi-Instrument Subscription and Custom Dialer Support

[dyamin]

• Added functionality to subscribe to order books of multiple instruments in a single request. Introduced an option to set a custom WebSocket dialer, which allows users to avoid overloading the default dialer. This is especially useful for those managing multiple exchanges.
• Fix data race: I identified a data race issue in the client. The race condition was specifically observed in the receiver and sender methods, concurrently accessing, and modifying the c.lastTransmit map without proper synchronization.
• resolves the issue of goroutine leaks in the client and public process methods.
• Handle channel operations without launching unnecessary goroutines

[amir-the-h]

@dyamin Your PR is much appreciated and has now been merged. Thanks for your contribution.