feat(postgres): allow specifying deferred mode on unique constraints

[B4nan]

Allows specifying deferMode on a unique constraint to get around issues with batch updates mainly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.74%. Comparing base (c7972e7) to head (e2992a2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5537   +/-   ##
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  Coverage   99.74%   99.74%           
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  Files         261      261           
  Lines       18096    18100    +4     
  Branches     3860     3864    +4     
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+ Hits        18050    18054    +4     
  Misses         46       46           

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[DASPRiD]

Only after working on #5641 I spotted that you worked on this PR. Looking at the source, it seems we have some overlap, though this is specifically handling manually created unique constraints, while my PR handles unique constraints created automatically for 1:1 relations. How should we proceed on this? :)

[B4nan]

i can merge yours first and release to stable, since its a fix. i am keeping this one for v6.3, which i'd like to start working on soon (this weekend probably), i also have #5482 which is waiting for the exact same reason, and have some other small things i want in there that will deserve a blog post