chore(): marked apis public

[Tony133]

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?

Issue Number: N/A

What is the new behavior?

Proposal:

Annotate user-facing APIs with @publicApi, since in the recently added "ConditionalModule" module the @publicApi annotation was added(see here: https://github.com/nestjs/config/blob/master/lib/conditional.module.ts#L5), it would be nice if all user-facing APIs in this package had the api public annotation.

Does this PR introduce a breaking change?

• Yes
• No

Other information

If there are private APIs that are not public, which I forgot, please let me know so I can update the PR.