feat: add response example

[lit26]

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
• 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?

@ApiNotFoundResponse(
	{
	description: 'Unauthorized.',
	type: HttpExceptionDTO,
	example: {
		message: 'Unauthorized',
		status: 401
	}
)

Cannot add example

Issue Number: #2260

What is the new behavior?

Will add example alongside $ref

Does this PR introduce a breaking change?

• Yes
• No

Other information

[shaunhurryup]

@kamilmysliwiec @micalevisk This PR has been proposed for over a year, why hasn't it been merged or received any response?

[insidedw]

We want you guys to have a code review quickly, because if it gets merged, our code will be simpler.

[kamilmysliwiec]

lgtm