Expose Deployment Approvals to internal Environments API

What does this MR do and why?

To support the frontend work in #342180 (closed), we need to expose deployment.approvals and approval.created_at to the internal Environments API. created_at is now also exposed to the public Deployments API because the Approval entity is reused.

How to set up and validate locally

Internal API change

No user-facing change here.

1. Go to Projects > Deployments > Environments
2. Look for environments.json in network requests inspector
3. Verify that deployments associated to the environments have an approvals attribute

Public API change

1. Set up Deployment Approvals with >1 approval required.
2. Create an approval
3. Use the Deployments API to get a specific deployment.
4. Verify that the approval has a created_at attribute

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.