feat(core): allow using SQL expressions with custom types

[B4nan]

Adds two methods to the custom type interface that allows to adjust the SQL fragment
used to select or update the value.

Example usage:

export class PointType extends Type<Point, string> {

  convertToDatabaseValue(value: Point): string {
    return `point(${value.latitude} ${value.longitude})`;
  }

  convertToJSValue(value: string): Point {
    const m = value.match(/point\((\d+(\.\d+)?) (\d+(\.\d+)?)\)/i);
    return new Point(+m[1], +m[3]);
  }

  convertToJSValueSQL(key: string) {
    return `ST_AsText(${key})`;
  }

  convertToDatabaseValueSQL(key: string) {
    return `ST_PointFromText(${key})`;
  }

  getColumnType(): string {
    return 'point';
  }

}

Closes #735