Manual ACK packet support

[xiaocq2001]

A solution for issue https://github.com/bytebeamio/rumqtt/issues/841 and issue https://github.com/bytebeamio/rumqtt/issues/609.

Note the example of async_manual_acks_v5 needs PR https://github.com/bytebeamio/rumqtt/pull/854 to make broker re-send publishes.

Type of change

New feature (non-breaking change which adds functionality)

• get_manual_ack to return manual ack packet where
  • set_reason/set_code to update reason code
  • set_reason_string to set reason string
  • set_user_properties to set user properties
• manual_ack/try_manual_ack to send the modified packet (PUBACK/PUBREC) returned by get_manual_ack

Checklist:

• [ ] Formatted with cargo fmt
• [ ] Make an entry to CHANGELOG.md if it's relevant to the users of the library. If it's not relevant mention why.

[xiaocq2001]

Are there any additional comments or feedback? If this solution is acceptable, could you please advise on the steps I can take to expedite the merging process?

[xiaocq2001]

Added some docs about the API usage. Added two different ack way in example code and detailed comments on them.

Please check.

[xiaocq2001]

Any comments?

[xiaocq2001]

Any comments on manual ACK with reason code support here?

[xiaocq2001]

Is there any chance to merge this PR? It appears to be relatively independent and should not affect other ongoing discussions.