feat: Add support for MATERIALIZED, EPHEMRAL and ALIAS expr columns in table definition

[BrentonAD]

Summary

The schema DSL has no way to declare a column with a MATERIALIZED expression. ClickHouse allows DEFAULT | MATERIALIZED | EPHEMERAL | ALIAS expr per column (docs). Currently only DEFAULT is supported.

MATERIALIZED is not interchangeable with DEFAULT:

• MATERIALIZED — always computed and stored; cannot be supplied on INSERT.
• DEFAULT — applied only when the value is omitted; can be overridden.

So there is currently no way to express a derived, insert-immutable column, e.g. Date Date MATERIALIZED toDate(Datetime).

Like-wise for EPHEMERAL and ALIAS

Current Behaviour:

Embedding it in the type str as a workaround:

{"name": "Date", "type": "Date MATERIALIZED toDate(Datetime)"}

emits valid DDL on the first migrate, but does not round-trip:

• introspect captures a column's default expression only when default_kind == "DEFAULT". A live MATERIALIZED column reports default_kind == "MATERIALIZED", so the expression is dropped and type is read back as the base Date.
• generate / drift then compare schema "Date MATERIALIZED toDate(Datetime)" vs live "Date" and an ALTER MODIFY COLUMN is emitted on every run.

Net: the workaround silently breaks drift detection for that column.

Proposed

• Add default_kind: Literal["DEFAULT", "MATERIALIZED", "ALIAS", "EPHEMERAL"] = "DEFAULT" (alias defaultKind) to ColumnDefinition. default holds the expression.
• Render as `col` <type> <default_kind> <expr>
• Drop introspect's default_kind == "DEFAULT" filter so all kinds round-trip.

Caveats

1. Expressions must render unquoted. MATERIALIZED/ALIAS/EPHEMERAL values are SQL expressions, hence they need the raw-expression path, or toDate(Datetime) gets emitted as a quoted string.
2. Keep existing snapshots stable. Introducing the kind into a column's comparison identity would make the first diff after upgrade rewrite every existing column. Treat an absent/DEFAULT kind as identical to today so unchanged columns don't churn.

Version chkit-py 0.2.0