Add attestations list view

What does this MR do and why?

This adds the list view for attestations. This is developed under the slsa_provenance_statement feature flag.

Each attestation links to its own show view, which will be implemented in #566595.

References

Resolves #566593.

Screenshots or screen recordings

State	Demo
When user does not have read permission or feature flag is disabled
When there are no attestations (provides a link to the SLSA Component from the CI/CD Catalog)
When there are attestations	attestations

How to set up and validate locally

1. Run the following on rails console to populate your project with attestations. The list defaults to 20 attestations per page.

    project = Project.find_by_full_path('project-path')
    build = project.builds.last
    data = URI("https://gitlab.com/gitlab-org/software-supply-chain-security/tools/sigstore-local/-/raw/main/sample.sigstore.json").read

    25.times do |i|
      temp_file = Tempfile.new("sample_#{i + 1}.sigstore.json")
      temp_file.write(data)
      temp_file.rewind
      attestation = SupplyChain::Attestation.create!(
        project: build.project,
        build_id: build.id,
        status: ['success', 'error'].sample,
        predicate_kind: "provenance",
        predicate_type: "https://slsa.dev/provenance/v1",
        subject_digest: SecureRandom.hex(32),
        file: temp_file
      )
    end

2. Go to /your-project/-/attestations to view the attestations list.

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