Add support for github.com/99designs/keyring-backed keybases

[alessio]

Introduce new Keybase implementation that can leverage
operating systems' built-in functionalities to securely store
secrets.

This chunk is extracted from @poldsam's original PR:

• Switch secret store to the keyring secret store #4754

Thanks: @alexanderbez for the careful review.

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5029 into master will decrease coverage by 0.22%.
The diff coverage is 50.4%.

@@            Coverage Diff            @@
##           master   #5029      +/-   ##
=========================================
- Coverage   55.02%   54.8%   -0.23%     
=========================================
  Files         291     294       +3     
  Lines       17841   18222     +381     
=========================================
+ Hits         9817    9986     +169     
- Misses       7317    7490     +173     
- Partials      707     746      +39

[fedekunze]

utACK. Pending Changelog entry

[alessio]

FTR Changelog entry has been added, will fix the conflicts shortly

[alexanderbez]

In light of #4941, we need to be cognizant of varying signing algos.

[alexanderbez]

@alessio instead of just creating private auxiliary functions that all the Keybase implementations leverage, wouldn't it be more adventagous to have a public "base" Keybase that all the KeyBase implementations embed? This would allow other applications to potentially implement their own not to mention it would improve testing.

The overhead to doing this is minimal afaict.

[alexanderbez]

ACK

[alessio]

Thanks for the thorough review @alexanderbez

[zmanian]

Woohoo