Adding custom keyword through ajv "unique" , need to access repository.

### Describe the bug

I am adding custom keyword "unique" in loopback 4. It will check whether the entered value is present in db or not.

How can I access repository or request context in "unique" keyword definition or validation.

### Logs

_No response_

### Additional information

_No response_

### Reproduction

https://ajv.js.org/guide/async-validation.html

Start with the AJV async-validation guide linked in the reproduction and the custom "unique" keyword described in the issue. Determine whether repository or request-context access is supported during LoopBack validation, then document the supported approach and its limitations.

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