refactor!: modify `ProofExpr::result_evaluate` to return `ColumnarValue` and remove `table_length` as arg

[iajoiner]

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Rationale for this change

In order to make ProofPlans composable it is necessary to allow one or more ProofPlans as the input to another. As such it is a great idea to remove table_length as an argument in ProofPlan::result_evaluate which in turn requires removal in ProofExpr.

What changes are included in this PR?

• Remove table_length from ProofExpr::result_evaluate and return ColumnarValue.
• Remove table_length from ProofPlan::result_evaluate.

[iajoiner]

This PR replaced https://github.com/spaceandtimelabs/sxt-proof-of-sql/pull/334