Add Scala 3-specific method for building custom schemas

[kyri-petrou]

Improves UX of building custom schemas significantly for Scala 3:

Scala 2:

case class Author(id: String, firstName: String, lastName: String)

implicit val authorSchema: Schema[Any, Author] = Schema.obj("Author"){ implicit attrs =>
  List(
    field("id")(_.id),
    field("fullName")(_ => "foo"),
  )
}

Scala 3:

case class Author(id: String, firstName: String, lastName: String)

given Schema[Any, Author] = Schema.custom("Author")(
  field("id")(_.id),
  field("fullName")(_ => "foo"),
)

Not too sure on the name though, custom implies that it can be used to build other kinds of schemas, but customObject feels a bit too verbose. Thoughts @ghostdogpr?

[ghostdogpr]

Since the original was obj, how about customObj?

[kyri-petrou]

Since the original was obj, how about customObj?

Done 👍