Add helper method for creating custom enum schemas

[kyri-petrou]

Currently, the only way for a user to create a custom enum schema is a bit verbose, and requires the user to look up the enum schema derivation in order to implement one:

trait Foo
object Foo {
  case object A extends Foo
  case object B extends Foo
}

implicit val fooSchema: Schema[Any, Foo] = new Schema[Any, Foo] {
  def toType(isInput: Boolean = false, isSubscription: Boolean = false): __Type =
    Types.makeEnum(
      Some("Foo"),
      None,
      List(__EnumValue("FooB", None, false, None, None),  __EnumValue("FooA", None, false, None, None)),
      None,
    )

  override def resolve(value: Foo): Step[R] = PureStep(StringValue("Foo" + value.toString))
}

With this PR, we add a helper method in the Schema companion object to make it easier to create one:

import Schema._

implicit val fooSchema: Schema[Any, Foo] = enumSchema[Foo](
  "Foo",
  values = List(enumValue("FooA"), enumValue("FooB")),
  repr = v => "Foo" + v.toString
)