Database Schema Definition API now supports creation of generated columns

The DB Schema Definition added support for generated columns, which can then be part of indexes just like regular ones.

References:
MySql: https://dev.mysql.com/doc/refman/9.7/en/create-table-generated-columns.html
MariaDb: https://mariadb.com/docs/server/reference/sql-statements/data-definition...
PostgreSql: https://www.postgresql.org/docs/current/ddl-generated-columns.html
SQLite: https://sqlite.org/gencol.html

• Column::generated() adds the definition of a generated table column.
• Drupal\Core\Database\SchemaDefinition\GeneratedColumnStorage enumerates the possible storage options for the generated column.

Example:

    $schemaDefinition = new SchemaDefinition(
      type: SchemaDefinitionType::Test,
      name: 'test_schema',
      tables: [
        new Table(
          name: 'test_table',
          description: 'Basic test table with a generated column.',
          columns: [
            Column::serial(
              name: 'id',
            ),
            Column::varcharAscii(
              name: 'name',
              description: "A person's name",
              length: 255,
              notNull: TRUE,
              default: new StringValue(''),
              binary: TRUE,
            ),
            Column::int(
              name: 'age',
              description: "The person's age",
              size: ColumnSize::Small,
              unsigned: TRUE,
              notNull: TRUE,
              default: new IntValue(0),
            ),
            Column::generated(
              name: 'double_age',
              type: ColumnType::Int,
              description: "The person's age X 2",
              storage: GeneratedColumnStorage::Stored,
              expression: new GeneratedColumnExpression('[age] * 2'),
            ),
            Column::generated(
              name: 'shout',
              type: ColumnType::Varchar,
              description: "The person's name in capitals",
              storage: GeneratedColumnStorage::Virtual,
              length: 512,
              expression: new GeneratedColumnExpression('UPPER([name])'),
            ),
            Column::generated(
              name: 'big_age',
              type: ColumnType::Int,
              description: "The person's age in a wider unsigned column",
              storage: GeneratedColumnStorage::Stored,
              size: ColumnSize::Big,
              unsigned: TRUE,
              expression: new GeneratedColumnExpression('[age] * 100'),
            ),
            Column::generated(
              name: 'half_age',
              type: ColumnType::Numeric,
              description: "Half of the person's age",
              storage: GeneratedColumnStorage::Virtual,
              precision: 10,
              scale: 2,
              expression: new GeneratedColumnExpression('[age] / 2'),
            ),
          ],
          primaryKey: new PrimaryKey(['id']),
          indexes: [
            new Index(
              name: 'double_age',
              columns: ['double_age'],
            ),
          ],
        ),
      ],
    );