providing a constraint property constant for dependency consistency

[grokspawn]

Description of the change:
add a convenience type for olm.constraint family of dependencies for consistent approach to dependency detection

Motivation for the change:
working in pursuit of operator-framework/operator-controller#779 identified that there are not constant types to represent all the dependency types in one place. This should help make operator-framework/operator-controller#781 cleaner.

They are currently divided across

1. alpha/property/property.go (for package/gvk); and
2. pkg/registry/types.go (for constraint)

This appears to be for a few reasons:

1. operator-registry doesn't enforce/convert olm.constraint; it's deferred until olm's resolver
2. operator-lifecycle-manager uses bare "olm.constraint" strings instead of using a const type
3. in alpha/property/property.go Parse() this property passes as opaque json.RawMessage type via the default case since it has no dedicated storage type in this context.

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[perdasilva]

lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grokspawn, m1kola, perdasilva

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[perdasilva]

/lgtm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.02%. Comparing base (aa0777c) to head (de1f5fa).
Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   54.02%   54.02%           
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  Files         108      108           
  Lines       11266    11266           
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  Hits         6087     6087           
  Misses       4190     4190           
  Partials      989      989           

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