fix(type): config service get and getOrThrow methods return type

[PongDev]

When a ConfigService type is provided, the return type should match the given type; otherwise, a compile error should occur.

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?

Issue Number: N/A

When the type of receiver variable and the type provided by ConfigService don't match, no error occurs.

What is the new behavior?

When a ConfigService type is provided, a typescript compile error will occur if return type and given type doesn't match.

Does this PR introduce a breaking change?

• Yes
• No

If a variable type has previously provided but doesn't match the return type provided in ConfigService, typescript will produce a compile error and // @ts-ignore is required to ignore error.

Other information