FlatlineRule: Extract functions that could be overriden by child classes

[rundef]

Description

In a project, I created a class extending FlatlineRule and added some custom code. However, I had to copy the code of check_for_match and garbage_function in the children class just to change a few lines of code, which isn't great.

This PR adds:

• a get_threshold function (users will be able to override that function in case the threshold changes depending on the key).
• a get_keys function (in my case I'm retrieving the keys from an external API)
• a get_event_data function (to add custom variable for alerts)

Checklist

• [x] I have reviewed the contributing guidelines.
• [ ] I have included unit tests for my changes or additions.
• [x] I have successfully run make test-docker with my changes.
• [x] I have manually tested all relevant modes of the change in this PR.
• [ ] I have updated the documentation.
• [x] I have updated the changelog.

Questions or Comments

I did not add any unit tests because there isn't any new feature. I did not update the documentation because there's no section about extending rules.