Closes #35641: Allow EPHEMERAL columns without explicit default expression

Why it matters

This feature enables users to define EPHEMERAL columns without specifying default expressions, simplifying table schema definitions and improving usability by reducing boilerplate.

How to use it

When creating or altering tables, users can declare columns with the EPHEMERAL modifier without providing a default expression. For example:

CREATE TABLE example (
    id UInt32,
    ephemeral_col Int32 EPHEMERAL
) ENGINE = ...;