feat(core): allow disabling duplicate entities discovery validation

[thiagomini]

This PR adds a new option to the discovery object called checkDuplicateEntities. This new option aims to allow the client code to control the behavior regarding duplicated entities discovered. There might be situations where we just want to ignore the duplicates.

Closes mikro-orm/nestjs#132

[rubiin]

looks good

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (79af75c) 99.64% compared to head (aeacd40) 99.64%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4618   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files         215      215           
  Lines       14247    14261   +14     
  Branches     3389     3394    +5     
=======================================
+ Hits        14196    14210   +14     
  Misses         50       50           
  Partials        1        1           

Files Changed	Coverage Δ
packages/core/src/utils/Configuration.ts	100.00% <ø> (ø)
packages/core/src/metadata/MetadataDiscovery.ts	98.99% <100.00%> (ø)
packages/core/src/metadata/MetadataValidator.ts	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[thiagomini]

Thanks for accepting and merging the PR, Martin! That will definitely help me :)