Add Filter for the `mitigation` Field in a Finding

[thekief]

Is your feature request related to a problem? Please describe

Most fields in the finding DTO can be filtered using the API or the UI. One of the fields that cannot be filtered is mitigation. We have quite some data that has this field set instead of fix_version, thus, it would be great to see a similar support for it,

Describe the solution you'd like

Following a similar format as the query parameter related to fix_version

I propose two new query parameters:

• mitigation_available
  • checks if a value, atleast 1 character, is set in the mitigation-field
• mitigation
  • filters the mitigation-field content
  • note: to _version suffix, as the mitigation might be something different than a suggested version

Describe alternatives you've considered

Querying the database.

Additional context

relates to #14556