Add apply method to Schema

[kyri-petrou]

This PR adds helper methods to "summon" a schema of type A. This is particularly useful in Scala 3 when schemas are derived via the derives keyword. e.g.,:

case class Foo(x: Int) derives Schema.SemiAuto
case class Bar(x: Int)

// Before
given Schema[Any, Bar] = summon[Schema[Any, Foo]].contramap(???)

// Now
given Schema[Any, Bar] = Schema[Foo].contramap(???)

Also works with custom schemas:

trait Env
object MySchema extends SchemaDerivation[Env]
case class Foo(x: Int) derives MySchema.SemiAuto
case class Bar(x: Int)

given Schema[Env, Bar] = MySchema[Foo].contramap(???)

Another option would be to have the apply method defined on object Schema directly as below, but I thought it might be a bit annoying having to add the environment parameter each time

def apply[R, A](implicit ev: Schema[R, A]): Schema[R, A] = ev

[frekw]

Nice improvement! 👍

[kyri-petrou]

Agh, should probably add one for ArgBuilder while at it. I'll add it now