Get Heptapod version by API

Currently, the API only contains the GitLab version (and the revision).

It could be useful to be able to retrieve the Heptapod version as well. A possible use-case would be to check if Heptapod is up-to-date on a dashboard.

I'm willing to work on this issue.

Files to edit (probably):

app/models/instance_metadata.rb
spec/models/instance_metadata_spec.rb
lib/api/version.rb

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