Feature req/ suggestion. Public API for client._out_messages.clear()

Minimal example:
```
def clear_messages(self) -> None:
with client._out_message_mutex:
client._out_messages.clear()
```

Power users can encounter situations in which delivery of customised QoS 1 or QoS 2 messages, e.g. using topic aliases fails. Without any way to clear undelivered messages and try again, these can cause the server to kick the client. If the user has called client.loop_forever, this creates an unnecessary connection cycle, that they must currently resort to calling Paho's private methods to resolve.

This is the simplest solution suggested by Josh last year in issue #893

Start at the client implementation containing _out_messages and _out_message_mutex, and trace how queued QoS 1 and QoS 2 messages are managed. Add the supported public clearing entry point described in the issue, then verify that undelivered messages can be cleared without requiring private methods.

Written by the indexing model from the issue text.