SLSA Attestation UI/UX

The purpose of this issue is to discuss the UI/UX elements to be added to allow users to discover and view attestations generated by the GitLab trusted control plane.

Below are the proposed components to be added:

1. Error state - messaging in the pipeline view to indicate an error was encountered when generating an attestation
2. Navigation - an element to enable users to find all attestations for a project
3. Attestations List View - a paginated, filterable list showing all attestations for a project
4. Attestations Show View - a page showing the details for a specific attestation and download it