feat(exports): Expose more of the OpenAPI types

[drewish]

I found myself having to make changes to the doc built by the module. It would have been helpful to have these types available.

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 only the OpenAPIObject type is exported.

Issue Number: N/A

What is the new behavior?

Now all the types in open-api-spec.interface are exported. This would allow others to use them without having to copy them into their own code.

Does this PR introduce a breaking change?

• Yes
• No

Other information

If possible, I'd love to backport this to 8.x