New Sbt code generation setting

[juanpedromoreno]

As part of this ticket, we need to add a new sbt setting to be able to specify custom imports including custom marshallers for serializing/deserializing data.

The @fedefernandez's proposal would be:

(Extensible) ADT (provided by frees-rpc atm):

abstract class MarshallersImport(val marshallersImport: String) extends Product with Serializable
case object BigDecimalAvroMarshallers extends MarshallersImport("import ....")
case object JavaTimeDateAvroMarshallers extends MarshallersImport("import ....")
case object JodaDateTimeAvroMarshallers extends MarshallersImport("import ....")

New Sbt Setting:

lazy val idlGenCodecImports: SettingKey[List[MarshallersImport]]

Default value:

List(BigDecimalAvroMarshallers, JavaTimeDateAvroMarshallers)

This new setting would bring the ability to define the custom user marshallers in the build.sbt.

• Sbt Settings.
• Docs.

Related to #370