fix(graphql): readonly arrays not allowed in decorator input

[CarsonF]

PR Checklist

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• [x] 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?

• [x] 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?

const interfaces = [A, B] as const;

@ObjectType({
  implements: interfaces, // error
})

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

[CarsonF]

minor changes to support both array and read only arrays

These represent the same thing. Mutable arrays are a superset of read-only ones. But if this style is preferred I can switch it.

[micalevisk]

I just noticed something that would break due to that ReadonlyArray:

const imports: ResolveFieldOptions['middleware'] = [];
imports.push(() => {})

because we can't invoke .push on readonly arrays

and I believe that we shouldn't prevent that usage