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ARROW-8789: [Rust] Add separate crate for integration test binaries

This PR simply adds a separate crate for integration test binaries with binaries that do nothing yet.

Since this crate is not one we ship, I think we can more freely accept PRs into this crate as we collaborate on the integration testing, and this will have lower overhead than maintaining a separate branch for integration testing, IMHO.

Closes #7168 from andygrove/it-crate

Authored-by: Andy Grove <andygrove@nvidia.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>