Add keys subcommands to sign and verify arbitrary text payloads.

[alessio]

Closes: #4581

• 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

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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 #5066 into master will increase coverage by 0.09%.
The diff coverage is 78.46%.

@@            Coverage Diff             @@
##           master    #5066      +/-   ##
==========================================
+ Coverage   55.03%   55.13%   +0.09%     
==========================================
  Files         291      293       +2     
  Lines       17838    17903      +65     
==========================================
+ Hits         9817     9870      +53     
- Misses       7314     7319       +5     
- Partials      707      714       +7

[alexanderbez]

Thanks, @alessio but I'm going to close this PR for now. There needs to be a broader discussion and agreement on arbitrary message signing in addition to integration with Ledger.

ref: cosmos/ibc#33

/cc @jackzampolin @cwgoes @zmanian