pkg/cache: configurable backend preference

[joelanford]

Description of the change:
Update the FBC cache library to accept a preferred backend.

NOTE: this does not (yet) change opm serve to expose this configuration knob in the CLI.

Motivation for the change:
Allow importers of operator-registry to construct a cache with a specific backend. This is a necessary step to enable catalog mutator tools to be able to use the input catalog's cache format when it writes the output catalog.

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

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 48.47%. Comparing base (2e0d6a2) to head (f3d6d3b).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/cache/cache.go	64.28%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1424      +/-   ##
==========================================
+ Coverage   48.23%   48.47%   +0.24%     
==========================================
  Files         136      136              
  Lines       12805    12814       +9     
==========================================
+ Hits         6176     6212      +36     
+ Misses       5594     5561      -33     
- Partials     1035     1041       +6     

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[perdasilva]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: joelanford, perdasilva

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