Add Standalone Secret Detection Service in the Blueprint

What does this MR do and why?

This MR adds the ADR entry for a standalone Secret Detection Scanner Service. The changes include in the following files:

• /doc/architecture/blueprints/secret_detection/decisions/004_secret_detection_scanner_service.md
• /doc/architecture/blueprints/secret_detection/img/004_rpc_service.jpg
• /doc/architecture/blueprints/secret_detection/index.md

Relevant Issue Numbers

• Technical Discovery: RPC Service to run Secret ... (#462359) • Vishwa Bhat • 17.2

MR acceptance checklist

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.