Add a new guardrail for whitelist fields based on the user input for WSO2 AI Gateway

### Problem

Currently, there is a guardrail for the blacklist(anonymized) fields and values using regex in WSO2 AI Gateway.
But there is no option to whitelist selected components and blacklist all other values.

### Proposed Solution

Add the whitelist guardrail

### Alternatives

_No response_

### Version

_No response_