feat: allow acknowledge through generic API

[gekmihesg]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
Allow acknowledging alerts through the generic API. This can be helpful for some services to stop unnecessary escalation.

Describe any introduced API changes:
/api/v2/generic/incoming?dedup=foo&action=acknowledge will now acknowledge matching alerts. action=close remains unchanged.