Add a pluggable AMQP WebSocket connector

## Summary

A public, object-safe WebSocket connector abstraction allows `azure_core_amqp` callers to supply WebSocket stream creation. The current `fe2o3-amqp-ws` connector remains the default.

## Motivation

PR #5031 directly constructs `fe2o3_amqp_ws::WebSocketStream` in `sdk/core/azure_core_amqp/src/fe2o3/connection.rs`. Unlike the HTTP transport, the AMQP WebSocket transport has no public replacement hook. Applications need an extension point for another WebSocket implementation without changing the transport-kind enum.

## Proposal

- The connector returns an erased asynchronous byte stream compatible with `AsyncRead`, `AsyncWrite`, `Debug`, `Send`, `Unpin`, and `'static`.
- `AmqpConnectionOptions`, the Event Hubs producer and consumer builders, and `RecoverableConnection` carry the connector through initial connections and reconnects.
- The built-in `fe2o3-amqp-ws` connector remains the default.
- A mock connector test covers the initial connection and recovery paths.

Found while reviewing #5031.

Start with sdk/core/azure_core_amqp/src/fe2o3/connection.rs, where the current fe2o3-amqp-ws WebSocketStream is constructed, then trace AmqpConnectionOptions, the Event Hubs producer and consumer builders, and RecoverableConnection. Verify how a mock connector can cover both initial connection and recovery paths; done means callers can supply an object-safe connector while the built-in connector remains the default.

Written by the indexing model from the issue text.