Add registry-api as submodule of registry deployment

[jordanpadams]

Summary

Add registry-api as a git submodule of the registry deployment repository so it can be versioned and deployed as part of the integrated stack.

Acceptance Criteria

• registry-api added as a git submodule in the registry repository
• Submodule reference points to the appropriate release/tag
• Docker Compose or deployment scripts updated to use the submodule
• Documentation updated to reflect the new submodule setup

Parent Issue

Part of #517 — Integrate Registry sub-components with Managed OpenSearch

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