Implement API endpoint to get the SSH public key from pull mirror configurations
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### Proposal
Expose an endpoint for Public Keys in Mirror configuration.
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Original blurb written to support with motivation:
```
Hi!
Working on automating the setup of import+pull mirroring of over 200+ repos from Bitbucket to GitLab.
I would prefer to use SSH auth and not password based authentication, setting up the mirrors seems easy enough, but I can’t find any endpoint where I can access the public key that has been generated during the setup? Going to need all the different keys made so that I can authorize them in Bitbucket.
When checking the settings, there is no API call there either, the ssh public key is already preloaded into the page DOM.
https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/views/projects/mirrors/_table_pull_row.html.haml#L20
```
Workaround:
Can be extracted via the rails console
```ruby
project_ids = [ 1, 2, 3 ]
project_ids.each do |pid|
proj = Project.find_by(id:pid)
puts "#{proj.name}: #{proj.import_data.ssh_public_key}"
end
```
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