Implements subscription options retain as published feature

[andsel]

Release notes

Implements subscription options retain as published feature

What does this PR do?

• Fixed publish of retained message to have the retained flag set.
• Fixed existing integration test to avoid interference in tests (ada1602)
• Added test cases to verify retain as published.
• Implemented retain as published to forward retain flag only when requested.

Why is it important/What is the impact to the user?

Implement "retain as published" feature

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works
• I have updated the Changelog if it's a feature or a fix that has to be reported

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates Implements Subscription Options #808

Use cases