fix:add graceful shutdown to courier handler

[Gibheer]

Courier would not stop with the provided Background handler.
This changes the methods of Courier so that the graceful package can be
used in the same way as the http endpoints can be used.

Related issue

#295
@aeneasr

Proposed changes

This adapts the API of courier to match the API of http servers and therefore allows the usage of graceful.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

Further comments

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Awesome, thank you!