Allow transform() to be called outside of entity class

[jnfeinstein]

This function does not require an EntityClass to accomplish the transform. Placing this outside of the EntityClass allows consumers to define transforms as extension functions. For example:

fun Column<Int>.asString() = transform({ it.toString() }, { it.toInt() })

This can then be used as:

object FooTable : Table("foo") {
  val number = integer("number").asString()
}

[Tapac]

Hi @jnfeinstein , thank you for a PR, can you please add the test for a new function as I'm not sure that using that column in pure DSL style will works well.