Implements subscription options retain as published feature

Release notes

Fixed publish of retained message to have the retained flag set.
Fixed existing integration test to avoid interference in tests (ada16022a48beb3ec7be876b7d98fc34922bf462)
Added test cases to verify retain as published.
Implemented retain as published to forward retain flag only when requested.

Implement "retain as published" feature

[x] My code follows the style guidelines of this project
[x] 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)~~
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have updated the Changelog if it's a feature or a fix that has to be reported