feat(offline): support const project with arbitrary expression for const projection

offline only supports `select `, it is better to support arbitrary expression in const projection, e.g `select fn(12)`.

The feature is generally ease the workflow of integration tests that for example want to test the correctness of a external registered function via `create function` statement.

The issue is about extending const projection in offline mode to support arbitrary expressions like `select fn(12)`. Look at the SQL parser and query planner for const projection handling, likely in the offline execution path. Check how external registered functions are integrated. The goal is to allow expressions in const projections for integration testing of user-defined functions.

索引モデルが issue の本文から書いたものです。