fix to use amino multisig pubkey at migration

[yun-yeo]

Summary of changes

fix #562

Use custom LegacyAminoPubkey for multisig account pub key.

To softly upgrade bombay-11 network, we add upgrade handler for pain name multisig-pubkey-migration. The handler will be automatically added at BeginBlocker of plan height to allow pre-upgrade.

Report of required housekeeping

• Github issue OR spec proposal link
• Wrote tests
• Updated API documentation (client/lcd/swagger-ui/swagger.yaml)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

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(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass

[codecov]

Codecov Report

Merging #560 (2c44cb2) into main (77caae9) will decrease coverage by 0.34%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##             main     #560      +/-   ##
==========================================
- Coverage   44.88%   44.54%   -0.35%     
==========================================
  Files         118      120       +2     
  Lines        6829     6924      +95     
==========================================
+ Hits         3065     3084      +19     
- Misses       3527     3601      +74     
- Partials      237      239       +2     

Impacted Files	Coverage Δ
app/app.go	0.65% <0.00%> (-0.02%)	⬇️
custom/auth/legacy/v040/store.go	0.00% <0.00%> (ø)
custom/auth/legacy/v040/migrate.go	23.75% <66.66%> (ø)