Fix performance issue in OpenAPI Schema validation logic

[imesh94]

Fix performance issue in OpenAPI Schema validation logic

This PR will add following changes to overcome the performance degrade observed after enabling OpenAPI schema validation.
- Reuse the already created validator for each API in schema validation.
- Get OpenAPI object from the message context instead of creating a new OpenAPI object.

Issue link: https://github.com/wso2-enterprise/wso2-apim-internal/issues/6782

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