feat: support selecting optional configs with selectConfig

[Nikaple]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: /~https://github.com/Nikaple/nest-typed-config/blob/main/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?

• Related issue linked using fixes #number
• 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?

Selecting optional config marked as @Optional() will throw error when config does not exist.

Issue Number: N/A

What is the new behavior?

Can provide an optional argument { allowOptional: true } to select optional config.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[github-actions]

🎉 This PR is included in version 2.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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