WIP: adopt keyring by default

[alessio]

Breaking changes:

• client/context/NewCLIContextWithFrom is gone
• client/context/GetFromFields now takes an extra io.Reader
• client/context/Context.WithFrom() now sets both FromName and FromAddress fields.

New client/Context's features:

• An input buffer should be passed via WithInput()
• WithKeybase() initialises a keybase/keybase according to flags or configuration
• WithFromFields() calls WithFrom() and passes the contents of the --from flag as argument

keys commands use new keyring functionalities by default.
Use --legacy-keybase to switch back to the old keybase.

---

• 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 #5132 into master will decrease coverage by 0.09%.
The diff coverage is 34.22%.

@@            Coverage Diff            @@
##           master    #5132     +/-   ##
=========================================
- Coverage   54.96%   54.87%   -0.1%     
=========================================
  Files         297      297             
  Lines       18231    18283     +52     
=========================================
+ Hits        10021    10032     +11     
- Misses       7472     7523     +51     
+ Partials      738      728     -10

[alessio]

Superseded by #5180
Hence closing.