Add new definition for sdk.Router, add WithRouter to the BaseApp

[PumpkinSeed]

Closes: #5455

Description

This PR makes the baseapp more generic. Add custom Router definitions opening it up for stateful routers.

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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 #5458 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5458      +/-   ##
==========================================
+ Coverage   54.38%   54.39%   +<.01%     
==========================================
  Files         313      313              
  Lines       18836    18838       +2     
==========================================
+ Hits        10244    10246       +2     
  Misses       7807     7807              
  Partials      785      785

Impacted Files	Coverage Δ
baseapp/baseapp.go	71.58% <100%> (+0.21%)	⬆️
baseapp/router.go	100% <100%> (ø)	⬆️

[sunnya97]

Thanks @PumpkinSeed! I actually had a finished PR for this already. #5452

Sorry, should have linked to it in the issue. Do you mind pointing the PR to the branch ctx_msg_router, so we can merge in the test you wrote?

[alexanderbez]

Thanks @PumpkinSeed! As @sunnya97 stated, this should be closed or pushed to @sunnya97's branch to include the additional tests you wrote. Thanks for the contributions!

[PumpkinSeed]

As discussion above.