feat(api-extension): add support for open api extensions via decorator (#195)

[tomchinery]

This commit adds support for Open API Extensions via a @APIextension decorator.
Usage:

@ApiExtension(<extensionName>, <extensionPropertyObject>)

Example:

@ApiExtension('x-foo', { hello: 'world' })

PR Checklist

Please check if your PR fulfills the following requirements:

• 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
[x] 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?

Issue Number: Solves #195

What is the new behavior?

@ApiExtension('x-foo', { hello: 'world' })

Adds an extension to the Swagger Specification:

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[andreoav]

@kamilmysliwiec Do you know when you can make this PR available?

[kamilmysliwiec]

@tomchinery thank you! Would you like to create a PR to the docs as well?

[tomchinery]

@kamilmysliwiec

@tomchinery thank you! Would you like to create a PR to the docs as well?

Sure, will do so now!

[kamilmysliwiec]

Amazing, thank you @tomchinery!

[tomchinery]

@kamilmysliwiec The docs PR is pushed up here: nestjs/docs.nestjs.com#1021.

Let me know if there's anything else needed. Thanks!

[kamilmysliwiec]

Thank you!

[elizabetht]

@tomchinery @kamilmysliwiec Thanks for adding this support for api extensions to swagger docs. How can I add multiple ApiExtensions? My company's swagger validation requires that I add 4 different x-foo: foo, x-bar: baz, x-something: somethingelse, x-ping: pong to my swagger specs under each path definition. But with this extension, I am able to only add only one api extension. I did the following but only the first one got picked up.

@ApiExtension("ping", "pong")
@ApiExtension("foo", "bar")

[kamilmysliwiec]

@elizabetht Please create a separate issue and share a minimum reproduction repository 🙌