Feature-gate datafusion-substrait behind optional feature to reduce compile time #21269
Description
Is your feature request related to a problem or challenge?
datafusion-substrait is always compiled as a dependency of datafusion-sqllogictest, even though most developers and CI jobs don't need substrait support. This adds unnecessary compile time.
Describe the solution you'd like
Make datafusion-substrait an optional dependency gated behind a substrait feature flag. Only the substrait round-trip test needs it.
Additional context
PR: #21268