Add web socket custom headers support in nats py client

[saurabhojha]

This pull request introduces support for custom headers in WebSocket connections to the NATS server. The key changes include:

Provision for supplying custom WebSocket request headers — developers can now attach additional HTTP headers (e.g., authorization tokens or tracing metadata) when establishing a WebSocket connection via NATS py.

A reference to a related issue in the .NET client repository (nats-io/nats.net#305) highlights the broader context and motivation for this enhancement.

Usage:
You can pass headers as a nats connection option ws_connection_headers.

nc = await nats.connect(
            "wss://localhost:8081",
            ws_connection_headers={
            "Authorization": ["Bearer SecureToken"],
            "X-TLS-Client": ["secure-client-v1"]
             }
        )