Updated will publishing to contains also the message expire property

Release notes

Updated will publishing to contains also the message expire property.

Change the publishing of will messages (PostOffice.publishWill method) to include also the message expiry property.

Completes the development to support message expiry, implementing the will publishing part.

[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