[Enh]: Add Pre-Processing Call Out to HTTP Endpoint

[JerryNixon]

Add a pre-processing step that calls an external HTTP endpoint before executing a request. DAB sends the incoming request payload to the endpoint, allowing custom logic to validate, modify, or reject the request before it reaches the database. This enables scenarios such as input validation, policy enforcement, enrichment, or business rules without requiring changes to the database schema or API surface.