Fix staging validator type change bug

[jribbink]

Description

Patch for missing type change handler, needed for NFT migrations/other custom type change rules.

Will make a follow up to export this from flow-go & use this, but this is a stop-gap.

https://discord.com/channels/613813861610684416/621847426201944074/1225634870730948689

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 42.04%. Comparing base (5c0fa11) to head (e888b63).

Files	Patch %	Lines
internal/migrate/staging_validator.go	53.33%	5 Missing and 2 partials ⚠️
internal/migrate/stage_contract.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##           feature/stable-cadence    #1495      +/-   ##
==========================================================
+ Coverage                   41.99%   42.04%   +0.05%     
==========================================================
  Files                          59       59              
  Lines                        3360     3375      +15     
==========================================================
+ Hits                         1411     1419       +8     
- Misses                       1745     1750       +5     
- Partials                      204      206       +2     

Flag	Coverage Δ
unittests	42.04% <50.00%> (+0.05%)	⬆️

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