Update to Substrait v0.87.0

[nielspardon]

• add new Plan.ExecutionBehavior
• add ExecutionContextVariable support (CURRENT_DATE, CURRENT_TIMESTAMP, CURRENT_TIMEZONE)
• use enum arg function signatures for std_dev and variance functions instead of function option signatures

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Release Notes

0.87.0 (2026-03-29)

Features

• extensions: add std_dev and variance with distribution enum arg (#1011) (00bc3c2)
• introduce ExecutionContextVariable and ExecutionBehaviour #945) (6b98c27)
• tests: add enum argument support to FuncTestCase grammar (#1010) (7b39d4c)

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see https://github.com/substrait-io/substrait/releases/tag/v0.87.0