feat: Add schema support for "example" annotations

[danielsharvey]

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?

Currently, the example annotation is not correctly copied into the schema object.

See https://swagger.io/docs/specification/adding-examples/.

Refer to:

swagger/lib/services/swagger-types-mapper.ts

Lines 114 to 124 in 02e0341

	private getSchemaOptions(param: Record<string, any>): Partial<SchemaObject> {
	const schemaKeys = this.getSchemaOptionsKeys();
	const optionsObject: Partial<SchemaObject> = schemaKeys.reduce(
	(acc, key) => ({
	...acc,
	[key]: param[key]
	}),
	{}
	);
	return omitBy(optionsObject, isUndefined);
	}

and

swagger/lib/services/swagger-types-mapper.ts

Lines 140 to 161 in 02e0341

	private getSchemaOptionsKeys(): Array<keyof SchemaObject> {
	return [
	'properties',
	'patternProperties',
	'additionalProperties',
	'minimum',
	'maximum',
	'maxProperties',
	'minItems',
	'minProperties',
	'maxItems',
	'minLength',
	'maxLength',
	'exclusiveMaximum',
	'exclusiveMinimum',
	'uniqueItems',
	'title',
	'format',
	'pattern',
	'nullable',
	'default'
	];

Issue Number: N/A

What is the new behavior?

Example annotations/properties are now correctly copied.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm