[kotlin-tooling-core] Allow nullable extra values

The change is binary compatible, but not source compatible.
Potential clients like Google might need to adapt
a potential `IdeaKotlinExtrasSerializationExtension` implementaiton.

Right now, no such implementation is known

libraries/tools/kotlin-gradle-plugin-idea/src/main/kotlin/org/jetbrains/kotlin/gradle/idea/serialize/IdeaKotlinExtrasSerializationExtension.kt

@@ -18,10 +18,10 @@ import org.jetbrains.kotlin.tooling.core.Extras
  * inside the implementation of the given [IdeaKotlinExtrasSerializer]
  */
 interface IdeaKotlinExtrasSerializationExtension {
-    fun <T : Any> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>?
+    fun <T> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>?
 
     object Empty : IdeaKotlinExtrasSerializationExtension {
-        override fun <T : Any> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>? = null
+        override fun <T> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>? = null
     }
 }
 
@@ -48,7 +48,7 @@ private class IdeaKotlinExtrasSerializationExtensionBuilderImpl : IdeaKotlinExtr
 private class IdeaKotlinExtrasSerializationExtensionImpl(
     private val map: Map<Extras.Key<*>, IdeaKotlinExtrasSerializer<*>>
 ) : IdeaKotlinExtrasSerializationExtension {
-    override fun <T : Any> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>? {
+    override fun <T> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>? {
         @Suppress("unchecked_cast")
         return map[key] as? IdeaKotlinExtrasSerializer<T>
     }

libraries/tools/kotlin-gradle-plugin-idea/src/main/kotlin/org/jetbrains/kotlin/gradle/idea/serialize/IdeaKotlinExtrasSerializer.kt

@@ -8,7 +8,7 @@ package org.jetbrains.kotlin.gradle.idea.serialize
 import kotlin.reflect.KClass
 
 
-interface IdeaKotlinExtrasSerializer<T : Any> {
+interface IdeaKotlinExtrasSerializer<T> {
     fun serialize(context: IdeaKotlinSerializationContext, value: T): ByteArray?
     fun deserialize(context: IdeaKotlinSerializationContext, data: ByteArray): T?
 

libraries/tools/kotlin-gradle-plugin-idea/src/testFixtures/kotlin/org/jetbrains/kotlin/gradle/idea/testFixtures/serialize/TestIdeaExtrasSerializationExtension.kt

@@ -16,7 +16,7 @@ object TestIdeaExtrasSerializationExtension : IdeaKotlinExtrasSerializationExten
     val ignoredStringKey = extrasKeyOf<String>("ignored")
     val anySerializableKey = extrasKeyOf<Any>("serializable")
 
-    override fun <T : Any> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>? = when {
+    override fun <T> serializer(key: Extras.Key<T>): IdeaKotlinExtrasSerializer<T>? = when {
         key == ignoredStringKey -> null
         key == anySerializableKey -> IdeaKotlinExtrasSerializer.javaIoSerializable<Any>()
         key.type == extrasTypeOf<String>() -> IdeaKotlinStringExtrasSerializer