add string method to gas meter

[Sladuca]

Closes: #6128

Description

Implemented string methods for basicGasMeter and infiniteGasMeter

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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.
• [n/a] Wrote unit and integration tests
• [n/a] Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• [n/a] 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 #6137 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6137      +/-   ##
==========================================
- Coverage   54.83%   54.82%   -0.01%     
==========================================
  Files         439      439              
  Lines       26494    26498       +4     
==========================================
  Hits        14527    14527              
- Misses      10970    10974       +4     
  Partials      997      997              

[alexanderbez]

Thanks @Sladuca!

[alexanderbez]

ACK 🎉

Great job @Sladuca

[tac0turtle]

ACK