[FEATURE] Support substrait 0.64

[johanpel]

Summary

Add compatibility with substrait 0.64 and its packaged substrait-prost types.

Motivation

Consumers using substrait 0.64 cannot pass their substrait::proto::Plan to substrait-explain, because 0.7 currently expects the distinct type generated by substrait 0.63.

Proposed Solution

Update substrait to 0.64, align pbjson-types to 0.9, and handle the new ExecutionContextVariable expression variant.

Alternatives Considered

Consumers must remain on substrait 0.63 or maintain an adapter/fork.

Additional Context

Substrait 0.64 moved its protobuf types to the packaged substrait-prost crate.