Add datafusion-rocksdb crate with basic layout and README

[Copilot]

Plan for adding datafusion-rocksdb crate:

• [x] Explore repository structure and understand existing crate patterns
• [x] Verify build and test process works
• [x] Create datafusion-rocksdb crate directory structure
• [x] Add Cargo.toml for datafusion-rocksdb crate
• [x] Add README.md for datafusion-rocksdb crate
• [x] Create basic lib.rs with module structure
• [x] Verify the new crate builds successfully
• [x] Run final tests to ensure no regressions
• [x] Fix CI to use project-specific Rust toolchain version
• [x] Update Rust toolchain to 1.86.0 for cargo-machete compatibility

Original prompt

Create a branch adding a new crate datafusion-rocksdb. This crate will provide udfs and datafusion extension points. Start with just adding the general layout and readme

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