docs: add comprehensive planning document for lookup plugins

Create detailed implementation plan for DVLS lookup plugins:
- devolutions.dvls.secret (field-specific retrieval)
- devolutions.dvls.secret_full (complete credential object)

Plan includes 5 phases covering foundation, implementation,
documentation, optimization, and CI/CD integration.

Related to: DEVOPS-4069

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: simondevolutions