Fix 4837 alter table alter column

[griffio]

fixes #4837

🚧 🐘 🍔 Working on fix

Allow AlterTableAlterColumnMixin extends SqlColumnDef to provide operations to TableInterfaceGenerator as it now becomes a ColumnDefMixin source - however it only seems viable for Column Type changes

Override SqlColumnDef members ( use the column_type declared in the ALTER TABLE or existing SqlColumnDef declared in CREATE TABLE ) to fall back to the current values if they are not being altered. Only column_type and column_name can be used with SqlColumnDef. ColumnConstraints are not used to drop/set nullability (use query column nullable) as there is no matching clause in ALTER TABLE.

FIXED - If a migration step 1 is ALTER TABLE test ALTER COLUMN first DROP NOT NULL and step 2 is ALTER TABLE test ALTER COLUMN first TYPE BIGINT; changing the type propagates the same NOT NULL/NULL - keep the current queryColumn.nullable value when not a null value - means it was set in another migration and queryColumn propagates the correct nullability
e.g queryColumn.copy(element = alterColumnTable, nullable = nullableColumn ?: queryColumn.nullable)

🦺 Performed more testing of local snapshot with column type changes and nullability changes

1.sqm

CREATE TABLE X(
    y INTEGER NOT NULL,
    t TEXT,
    a INTEGER NOT NULL,
    b TEXT
);

2.sqm

ALTER TABLE X
ALTER COLUMN a TYPE BIGINT;

3.sqm

ALTER TABLE X
ALTER COLUMN y TYPE BIGINT;

4.sqm

ALTER TABLE X
ALTER COLUMN y DROP NOT NULL;

5.sqm

ALTER TABLE X
ALTER COLUMN t SET NOT NULL;

results in

public data class X(
  public val y: Long?,
  public val t: String,
  public val a: Long,
  public val b: String?,
)

[AlecKazakova]

looks great, thank you!