Handle multiple OpenAPI decorators on a single handler

[epiphone]

Handle cases such as

  @OpenAPI({
    summary: 'This value will be overwritten!',
    description: 'This value will remain'
  })
  @OpenAPI({
    summary: 'This value will remain'
  })
  listUsers() {
    // ...
  }

Previously the latter OpenAPI was ignored. With this PR, both decorators are applied top-down.

This also enables using OpenAPI internally in custom decorators, as needed in #9.

[codecov]

Codecov Report

Merging #10 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   99.25%   99.27%   +0.01%     
==========================================
  Files           4        4              
  Lines         135      137       +2     
  Branches       31       31              
==========================================
+ Hits          134      136       +2     
  Misses          1        1

Impacted Files	Coverage Δ
src/decorators.ts	100% <100%> (ø)	⬆️

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