[Scala 3] Field derivation from case class methods

[kyri-petrou]

Currently, derivation of graphql fields works only with case class arguments. With this PR, we add a new annotation (available for Scala 3 only) which allows us to annotate a method to include in field derivation. This is useful in cases that a field can be resolved from the values of other fields, e.g.,

case class Stock(apples: Int, oranges: Int) {
  @GQLField def total: Int = apples + oranges
}

The methods can also be annotated with any other annotation that is normally used to annotate case class fields:

case class Stock(apples: Int, oranges: Int) {
  @GQLField
  @GQLDescription("The total number of fruits in stock")
  def total: Int = apples + oranges
}

Derivation via this case class methods relies partially on Java runtime reflection. This is mostly because the macros required for compiler-based derivation are extremely complex and very hard to maintain. This means that this annotation should NOT be used when Ahead-of-time compilation is required (e.g., GraalVM native-image).

Regarding Scala 2: We could potentially add this method to Scala 2 as well, but we'd have to port the Scala 3 macro to Scala 2 to make this work. Also I'm not too sure whether it's even possible given that we use magnolia for the derivation

PS: I'm not sure regarding the naming of the annotation. Another option is to use @GQLInclude (as opposite of GQLExclude) - @ghostdogpr any thoughts on the naming?)