fix: support for multiple api decorators on schema class

[arabold]

Support multiple @ApiExtension decorators on the schema class. This implementation applies the same fix as #1525 but also for the class, not just for the member functions.

This enables you do use multiple @ApiExtension (and other mixed decorators) on an API schema definition like this:

@ApiExtension('x-foo', { test: 'test' })
@ApiExtension('x-bar', { test: 'test' })
class CatDto {
}

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Currently it is not possible to use the same decorator multiple times on the same class. I.e. using two @ApiExtension() decorators will only result in the first one being applied.

Issue Number: N/A

What is the new behavior?

This PR fixes this behavior and applies all decorators as expected. It implements the same fix as #1525 but on a class level.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM