Feat/add plan artifacts input

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

This MR adds a plan_artifacts input, allowing additional artifacts to be uploaded alongside the plan json.
We're using this to upload zips created by archive_file data blocks, allowing us to apply the plan using zips generated by tofu plan as part of a multi-step build pipeline.

Addresses the core concern in #102 (closed)