fix(plugin): call assignment function to add enum property by @IsEnum decorator

[seonggukchoi]

PR Checklist

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• 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)
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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 the option classValidatorShim is enabled, I expected the CLI to add an enum property into Swagger documentation automatically about a @IsEnum decorator used property, but it doesn't run expectation.
Then I tried using a @IsIn decorator instead of @IsEnum, and the behavior ran as expected.

Issue Number: N/A

What is the new behavior?

When the option classValidatorShim is enabled and using the @IsEnum decorator at a property, the CLI plugin adds the enum property automatically.

Does this PR introduce a breaking change?

• Yes
• No

Other information

If I need then I will create an issue with the information, I don't know why exists the readonly option, and why can't run adding enum options by the @IsIn decorator when enabled the option readonly.