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

[seonggukchoi]

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 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.

[kamilmysliwiec]

Could you update tests too?

[seonggukchoi]

Hi, @kamilmysliwiec. I haven't been able to pay attention to this PR recently.
I have read some of the test files that seem to be related to the model-class.visitor.ts file, but I couldn't intuitively understand them.
So, I'm not sure which files to add the tests to and how to add them.
Could you please help me add tests?
If there exists any guide on writing tests in this project, documentation explaining the module structure, similar tests, or anything else would be helpful.