Add GET /projects/:id/remote_mirrors/:mirror_id/public_key API endpoint

What does this MR do and why?

Adds the "GET /projects/:id/remote_mirrors/:mirror_id/public_key" API endpoint that extends the existing UI ability to obtain a remote mirrors public key to the Remote mirrors API.

• Repository mirroring - Get your SSH public key

Changelog: changed

References

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• #514005

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Before	After

How to set up and validate locally

1. Create mirror with SSH authentication
2. List a project’s remote mirrors to obtain the created remote mirror's ID
3. Make a request to the new endpoint. Example:

curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/42/remote_mirrors/101486/public_key"