Proof of concept: Add implementation for computing the required bytes to encode a message

This commit introduces a new API which computes the required bytes to
encode a message without actually serializing a message. By extension,
this API is meant to be used as a cornerstone for implementing Kotlin#2075.

Signed-off-by: George Papadopoulos <[email protected]>

Author: GeorgePap-719

formats/protobuf/api/kotlinx-serialization-protobuf.api

@@ -21,6 +21,10 @@ public final class kotlinx/serialization/protobuf/ProtoBufKt {
 	public static synthetic fun ProtoBuf$default (Lkotlinx/serialization/protobuf/ProtoBuf;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Lkotlinx/serialization/protobuf/ProtoBuf;
 }
 
+public final class kotlinx/serialization/protobuf/ProtoBufSerializedSizeCalculatorKt {
+	public static final fun getOrComputeSerializedSize (Lkotlinx/serialization/protobuf/ProtoBuf;Lkotlinx/serialization/SerializationStrategy;Ljava/lang/Object;)I
+}
+
 public final class kotlinx/serialization/protobuf/ProtoIntegerType : java/lang/Enum {
 	public static final field DEFAULT Lkotlinx/serialization/protobuf/ProtoIntegerType;
 	public static final field FIXED Lkotlinx/serialization/protobuf/ProtoIntegerType;

formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/ProtoBuf.kt

@@ -5,9 +5,9 @@
 package kotlinx.serialization.protobuf
 
 import kotlinx.serialization.*
-import kotlinx.serialization.modules.*
+import kotlinx.serialization.modules.EmptySerializersModule
+import kotlinx.serialization.modules.SerializersModule
 import kotlinx.serialization.protobuf.internal.*
-import kotlin.js.*
 
 /**
  * Implements [encoding][encodeToByteArray] and [decoding][decodeFromByteArray] classes to/from bytes
@@ -168,4 +168,4 @@ public class ProtoBufBuilder internal constructor(proto: ProtoBuf) {
 
 @ExperimentalSerializationApi
 private class ProtoBufImpl(encodeDefaults: Boolean, serializersModule: SerializersModule) :
-    ProtoBuf(encodeDefaults, serializersModule)
+    ProtoBuf(encodeDefaults, serializersModule)