feat: add support for unresolved expressions

[malinjawi]

Refs #515

This adds minimal protocol support for partially bound / unresolved expressions in Substrait.

Changes included:

• add Type.Unknown as a placeholder type for partially bound expressions
• add Expression.NamedExpression for unresolved name-based references
• document how partially bound expressions are detected structurally
• add textproto examples for unresolved Expression and ExtendedExpression
• update the type grammar to parse unknown
• regenerate the checked-in parser artifacts
• add validation and contract tests for the new examples

This keeps the design minimal:

• there is no separate unbound-expression hierarchy
• partially bound state is represented structurally by the presence of unknown and/or NamedExpression

Validation:

• buf lint
• buf generate
• pytest coverage for the new examples and parser/type handling

This PR adds representational support only. Consumers still need to bind unresolved names, types, and function references before execution unless they define unresolved-expression semantics themselves.

Note: the large generated diffs are from regenerating the checked-in ANTLR parser artifacts after adding unknown to the grammar.

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This change is 

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