feat(postgres): allow defining deferred FK constraints

[DASPRiD]

This PR adds the option to specify constraints as deferrable for relations.

On OneToOne and ManyToOne relations you can now specify a deferMode property, which can be either not deferrable (default), immediate or deferred. Those are also covered by a new DeferMode enum.

Closes #5306

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.71%. Comparing base (0e18346) to head (300caa2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #5384    +/-   ##
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  Coverage   99.71%   99.71%            
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  Files         222      222            
  Lines       17049    17061    +12     
  Branches     3967     4195   +228     
========================================
+ Hits        17000    17013    +13     
+ Misses         49       48     -1     

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[B4nan]

nice, thanks!

can you please also test schema diffing? either in the test you added or by updating this one:

https://github.com/mikro-orm/mikro-orm/blob/master/tests/features/schema-generator/fk-diffing.postgres.test.ts

[B4nan]

all right, thanks again!