Fix: MQTTv5 keep-alive duration <= 5 sec allowed

[Jathn]

This pull request removes the minimum requirement of 5 seconds of keep-alive for MQTTv5. Additionally, the case where the keep-alive is 0 is implemented for MQTTv5 as described in the standard.

The duration is now set with u16 instead of Duration as discussed in #964. edit: the duration set also changed in optional added commit 691912f for v4 It is similar to pull request #655 which changed the minimum requirement and 0-case handling.

Issue: #964 Attribute: This PR adds support for this in MQTTv5, following the approach used in @danieldougherty 's implementation for MQTTv4 in PR #655.

Type of change

Bug fix (non-breaking change which fixes an issue)

Checklist:

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