Implement MigrateDatabase RPC

[Warashi]

What this PR does:

as title

Why we need it:

When developing pipedv1, we have to use data with new fields, such as deploy target, or we have to patch pipedv1 temporarily to use the old field.
This PR implements the server side.

Follows #5476

Which issue(s) this PR fixes:

Part of #4980
Part of #5252

Does this PR introduce a user-facing change?: No

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 34 lines in your changes missing coverage. Please review.

Project coverage is 26.21%. Comparing base (786bf3d) to head (4519815).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/server/grpcapi/api.go	0.00%	21 Missing ⚠️
pkg/datastore/datastoretest/datastore.mock.go	0.00%	8 Missing ⚠️
pkg/datastore/applicationstore.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5497      +/-   ##
==========================================
- Coverage   26.23%   26.21%   -0.02%     
==========================================
  Files         460      460              
  Lines       49370    49404      +34     
==========================================
  Hits        12952    12952              
- Misses      35389    35423      +34     
  Partials     1029     1029              

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[ffjlabo]

LGTM