Add online GC configurations to registry chart
What does this MR do?
This MR exposes the registry gc configuration section, which is used to customize the behavior of online garbage collection. This feature requires the metadata database to be enabled.
This is an experimental feature, which will be first used for the GitLab.com registry.
Checklist
See Definition of done.
For anything in this list which will not be completed, please provide a reason in the MR discussion.
Required
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Merge Request Title and Description are up to date, accurate, and descriptive 
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MR targeting the appropriate branch 
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MR has a green pipeline on GitLab.com 
Expected (please provide an explanation if not completing)
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Test plan indicating conditions for success has been posted and passes 
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Documentation created/updated 
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Tests added 
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Integration tests added to GitLab QA 
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Equivalent MR/issue for omnibus-gitlab opened 
Edited  by DJ Mountney