Add Extensions metadata Decorator

[hihuz]

No description provided.

[hihuz]

Sorry, didn't mean to create the PR this early, closing for now.

[codecov]

Codecov Report

Merging #512 into master will decrease coverage by 0.46%.
The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
- Coverage   95.18%   94.72%   -0.47%     
==========================================
  Files          76       77       +1     
  Lines        1330     1346      +16     
  Branches      257      259       +2     
==========================================
+ Hits         1266     1275       +9     
- Misses         61       68       +7     
  Partials        3        3

Impacted Files	Coverage Δ
src/schema/schema-generator.ts	96.74% <ø> (ø)	⬆️
src/decorators/index.ts	100% <100%> (ø)	⬆️
src/decorators/Extensions.ts	42.85% <42.85%> (ø)
src/metadata/metadata-storage.ts	97.98% <62.5%> (-2.02%)	⬇️

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[MichalLytek]

@hihuz Feel free to reopen when it's ready

[hihuz]

I ended up creating a different PR, hope you don't mind: #521 🙂