feat: Add metadata to reports

[agdimech]

Allow rules to specify arbitrary metadata to be included in generated reports.

This is an extension point to allow rule authors to be able to add additional metadata to rules that are triggered. Examples of this include custom categorisations i.e: Severity.

Overloading explanation or info doesn't seem like the right place for these types of dimensions and thought it would be better to include a new "catch-all" field for these purposes.

[dontirun]

Hey! Can you describe the use case more in depth? Why add a metadata field instead of using something like the explanation or info fields?

[agdimech]

Hey! Can you describe the use case more in depth? Why add a metadata field instead of using something like the explanation or info fields?

Updated the description.

[dontirun]

Closing due to inactivity