Add FlagKeyringBackend to QueryTxsByEventsCmd

[jgimeno]

Closes: #XXX

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• 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, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5957 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #5957      +/-   ##
==========================================
- Coverage   56.19%   56.18%   -0.01%     
==========================================
  Files         341      341              
  Lines       20430    20432       +2     
==========================================
  Hits        11480    11480              
- Misses       8080     8082       +2     
  Partials      870      870              

[alexanderbez]

@jgimeno, for future reference, please try to remember to add all appropriate labels 👍 and use the PR template as best you can.