feat: add function for enum types

[slukes]

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?

When using enum values that are dynamic, the value returned depends on the order that modules are imported in

Issue Number: #2786

What is the new behavior?

enum can be a function which is called at the moment that the document is generated.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[slukes]

Hey @kamilmysliwiec I see you closed the associated issue. Did you mean to approve the PR at the same time?

[slukes]

@kamilmysliwiec up 😄

[kamilmysliwiec]

lgtm