add a parser option to validate oneofs

The exception looks like:
```
scalapb.json4s.JsonFormatException: Overlapping keys in oneof: primitive, wrapper
```

Author: sttawm

src/main/scala/scalapb/json4s/JsonFormat.scala

@@ -461,6 +461,7 @@ class Printer private (config: Printer.PrinterConfig) {
 object Parser {
   private final case class ParserConfig(
       isIgnoringUnknownFields: Boolean,
+      failOnOverlappingOneofKeys: Boolean,
       mapEntriesAsKeyValuePairs: Boolean,
       formatRegistry: FormatRegistry,
       typeRegistry: TypeRegistry
@@ -472,6 +473,7 @@ class Parser private (config: Parser.ParserConfig) {
     this(
       Parser.ParserConfig(
         isIgnoringUnknownFields = false,
+        failOnOverlappingOneofKeys = false,
         mapEntriesAsKeyValuePairs = false,
         JsonFormat.DefaultRegistry,
         TypeRegistry.empty
@@ -490,6 +492,7 @@ class Parser private (config: Parser.ParserConfig) {
     this(
       Parser.ParserConfig(
         isIgnoringUnknownFields = false,
+        failOnOverlappingOneofKeys = false,
         mapEntriesAsKeyValuePairs = false,
         formatRegistry,
         typeRegistry
@@ -499,6 +502,9 @@ class Parser private (config: Parser.ParserConfig) {
   def ignoringUnknownFields: Parser =
     new Parser(config.copy(isIgnoringUnknownFields = true))
 
+  def failOnOverlappingOneofKeys: Parser =
+    new Parser(config.copy(failOnOverlappingOneofKeys = true))
+
   def mapEntriesAsKeyValuePairs: Parser =
     new Parser(config.copy(mapEntriesAsKeyValuePairs = true))
 
@@ -527,7 +533,24 @@ class Parser private (config: Parser.ParserConfig) {
       value: JValue,
       skipTypeUrl: Boolean
   )(implicit cmp: GeneratedMessageCompanion[A]): A = {
-    cmp.messageReads.read(fromJsonToPMessage(cmp, value, skipTypeUrl))
+    val message = fromJsonToPMessage(cmp, value, skipTypeUrl)
+    if (config.failOnOverlappingOneofKeys) {
+      validateOneofs(message)
+    }
+    cmp.messageReads.read(message)
+  }
+
+  private def validateOneofs[A <: GeneratedMessage](message: PMessage) = {
+    message.value.keys
+      .groupBy(_.containingOneof)
+      .filter(x => x._1.isDefined && x._2.size != 1)
+      .values
+      .headOption
+      .foreach(keys =>
+        throw new JsonFormatException(
+          s"Overlapping keys in oneof: ${keys.map(_.name).mkString(", ")}"
+        )
+      )
   }
 
   private def fromJsonToPMessage(

src/test/scala/scalapb/json4s/JavaAssertions.scala

@@ -31,6 +31,13 @@ class IgnoringUnknownParserContext {
   )
 }
 
+class StrictOneofParserContext {
+  implicit val pc: ParserContext = ParserContext(
+    new Parser().failOnOverlappingOneofKeys,
+    JavaJsonFormat.parser // by default the java parser fails on overlapping keys
+  )
+}
+
 trait JavaAssertions {
   self: AnyFlatSpec with Matchers =>
 

src/test/scala/scalapb/json4s/JsonFormatSpec.scala

@@ -7,6 +7,7 @@ import com.google.protobuf.any.{Any => PBAny}
 import com.google.protobuf.util.JsonFormat.{TypeRegistry => JavaTypeRegistry}
 import com.google.protobuf.util.{JsonFormat => JavaJsonFormat}
 import jsontest.custom_collection.{Guitar, Studio}
+import jsontest.oneof.OneOf
 import jsontest.test._
 import jsontest.test3._
 import org.json4s.JsonDSL._
@@ -674,6 +675,19 @@ class JsonFormatSpec
     JsonFormat.parser.fromJsonString[TestAllTypes](javaJson) must be(obj)
   }
 
+  "oneofs" should "fail for overlapping keys if failOnOverlappingOneofKeys" in new StrictOneofParserContext {
+    val extraKey = """{"primitive": "", "wrapper": ""}"""
+    assertFails(extraKey, OneOf)
+  }
+
+  "oneofs" should "not fail for overlapping keys if failOnOverlappingOneofKeys with a scala parser" in new DefaultParserContext {
+    val extraKey = """{"primitive": "", "wrapper": ""}"""
+    val parsedScala = pc.scalaParser.fromJsonString[OneOf](extraKey)(OneOf)
+    parsedScala must be(
+      OneOf(field = OneOf.Field.Primitive(""))
+    )
+  }
+
   "TestProto" should "be TestJsonWithMapEntriesAsKeyValuePairs when converted to Proto with mapEntriesAsKeyValuePairs setting" in {
     JsonFormat.printer.mapEntriesAsKeyValuePairs.toJson(TestProto) must be(
       parse(TestJsonWithMapEntriesAsKeyValuePairs)