feat: add model signer

[jonburdo]

This implements OMS signing to produce a signal .sig file for any repo or collection of files.

Description

How Has This Been Tested?

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages
• Automated tests are provided as part of the PR for major new functionalities; testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.
• For first time contributors: Please reach out to the Reviewers to ensure all tests are being run, ensuring the label ok-to-test has been added to the PR.

If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
• Verify that UI/UX changes conform the UX guidelines for Kubeflow.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign ederign for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[tarilabs]

note: we should try to bump this in-toto-attestation to >1.0.0 if possible (I dont' see any lockfile for rh-image-signing)