Add support for custom requirements

**What would you like to be added**:
Support the same ability as `cosign -r ` to override the default designated requirements.

No files, tests, or entry points are named in the issue. Start by locating how designated requirements are currently selected and how command-line options are handled, then compare the requested behavior with `cosign -r `. Done means custom requirements can override the defaults and the behavior is covered by tests.

Written by the indexing model from the issue text.