Handle ServiceMsg in StdTxBuilder.SetMsgs

[blushi]

Description

closes: #7653

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #7731 into master will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #7731      +/-   ##
==========================================
- Coverage   54.31%   54.31%   -0.01%     
==========================================
  Files         610      610              
  Lines       38612    38621       +9     
==========================================
+ Hits        20972    20976       +4     
- Misses      15503    15508       +5     
  Partials     2137     2137              

[aaronc]

ACK