feat: Add directory input to Maven builder

[AdamKorcz]

Adds another input for the Maven builder to allow the user to specify the project directory. The current problem this solves is to make slsa-framework/example-package#253 work.

This was suggested by @laurentsimon in slsa-framework/example-package#253 (comment)

[laurentsimon]

Thanks. You could do some verification on the path to make sure it's under the workspace, something like https://github.com/slsa-framework/slsa-github-generator/blob/main/.github/actions/secure-download-artifact/action.yml#L42-L48

[AdamKorcz]

@laurentsimon Could you have another look, please?

[ianlewis]

Thanks. You could do some verification on the path to make sure it's under the workspace, something like https://github.com/slsa-framework/slsa-github-generator/blob/main/.github/actions/secure-download-artifact/action.yml#L42-L48

TBF, I believe the chdir would happen in the "untrusted" build's job anyway. So I don't think this is super important (though necessary if it's used elsewhere).

[laurentsimon]

@AdamKorcz friendly ping to update this PR

[AdamKorcz]

@AdamKorcz friendly ping to update this PR

Thanks for the ping! Resolved the comments.