Add update fee token gov handler to app.go

[sunnya97]

Closes: #714

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov-commenter]

Codecov Report

Merging #731 (a6e0303) into main (ac37680) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #731      +/-   ##
==========================================
- Coverage   18.73%   18.73%   -0.01%     
==========================================
  Files         171      172       +1     
  Lines       23901    23908       +7     
==========================================
  Hits         4478     4478              
- Misses      18649    18655       +6     
- Partials      774      775       +1     

Impacted Files	Coverage Δ
x/txfees/handler.go	0.00% <0.00%> (ø)

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