test: Test websocket changes

[purnimavenkatasubbu]

- What I did
Uptake of WebSocket changes from at_lookup in at_client.
- How I did it

• Renamed from getSocket() to underlying
• Added an optional parameter "atConnectionFactory" defaulted to SecureSocketFactory in the Remote secondary
• Added an optional parameter "atConnectionFactory" in syncService, NotificationService, at_client_impl and monitor.dart and defaulted that to secureSocketFactory

- How to verify it

• All the existing tests should pass
• Added new functional test "at_lookup_socket_test" which tests WebSocketFactory and SecureSocketFactory.

- TODO

• Refactor the existing at_functional_tests and at_end2end_tests to run with both securesocket and websocket changes.

- Description for the changelog