Manual ACK packet support

[xiaocq2001]

A solution for issue #841 and issue #609.

Note the example of async_manual_acks_v5 needs PR #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.