Add ability to configure Sigmax return state

[bartjkdp]

Description

This PR makes it possible to configure the Sigmax return state. Currently the status in Signalen always returns to "done external" (Extern: afgehandeld), but some municipalities would like to be able to set the status to "o" (Afgehandeld).

Checklist

• Keep the PR, and the amount of commits to a minimum
• The commit messages are meaningful and descriptive
• The change/fix is well documented, particularly in hard-to-understand areas of the code / unit tests
• Are there any breaking changes in the code, if so are they discussed and did the team agreed to these changes
• Check that the branch is based on main and is up to date with main
• Check that the PR targets main
• There are no merge conflicts and no conflicting Django migrations
• PR was created with the "Allow edits and access to secrets by maintainers" checkbox checked

How has this been tested?

• Provided unit tests that will prove the change/fix works as intended
• Tested the change/fix locally and all unit tests still pass
• Code coverage is at least 85% (the higher the better)
• No iSort, Flake8 and SPDX issues are present in the code

[bartjkdp]

We are currently testing this change in Leiden.