Spark: support CALL older_than function expressions

[wangyum]

Why are the changes needed?

CALL ... expire_snapshots should support function-based timestamp arguments (for example, current_timestamp() - INTERVAL 1 DAYS) forms across Spark versions.

What changes were proposed in this pull request?

Spark 3.4 / 3.5

• Extended Iceberg SQL extension grammar to allow function-call expressions in CALL arguments with optional interval subtraction:
  • functionCallExpression (MINUS intervalLiteral)?
• Added INTERVAL token/non-reserved handling in the extension grammar.
• Added parser coverage in TestCallStatementParser for:
  • current_timestamp()
  • current_timestamp() - INTERVAL 2 DAYS
  • mixed-case SQL + lowercase interval usage
  • negative parser boundaries for unsupported interval forms
• Added procedure coverage in TestExpireSnapshotsProcedure for:
  • current_timestamp() positive path
  • current_timestamp() - INTERVAL 2 DAYS positive path
  • mixed-case/lowercase SQL positive path

Spark 4.0 / 4.1

• Added procedure coverage in TestExpireSnapshotsProcedure for:
  • current_timestamp() positive path
  • mixed-case SQL + function-call positive path
  • interval subtraction and unsupported interval forms as negative cases
• Added a helper assertion to validate unsupported older_than expression failures consistently.
• Kept extension grammar changes aligned with 3.x for cross-version parity.

Closes #16057.