CORDA-973 Compression support for serialization (#2473)

* Serialization magic is now 7 bytes
* Introduce encoding property and whitelist

core/src/main/kotlin/net/corda/core/serialization/SerializationAPI.kt

@@ -1,5 +1,6 @@
 package net.corda.core.serialization
 
+import net.corda.core.DoNotImplement
 import net.corda.core.crypto.SecureHash
 import net.corda.core.crypto.sha256
 import net.corda.core.serialization.internal.effectiveSerializationEnv
@@ -99,14 +100,22 @@ abstract class SerializationFactory {
     }
 }
 typealias SerializationMagic = ByteSequence
+@DoNotImplement
+interface SerializationEncoding
+
 /**
  * Parameters to serialization and deserialization.
  */
+@DoNotImplement
 interface SerializationContext {
     /**
      * When serializing, use the format this header sequence represents.
      */
     val preferredSerializationVersion: SerializationMagic
+    /**
+     * If non-null, apply this encoding (typically compression) when serializing.
+     */
+    val encoding: SerializationEncoding?
     /**
      * The class loader to use for deserialization.
      */
@@ -115,6 +124,10 @@ interface SerializationContext {
      * A whitelist that contains (mostly for security purposes) which classes can be serialized and deserialized.
      */
     val whitelist: ClassWhitelist
+    /**
+     * A whitelist that determines (mostly for security purposes) whether a particular encoding may be used when deserializing.
+     */
+    val encodingWhitelist: EncodingWhitelist
     /**
      * A map of any addition properties specific to the particular use case.
      */
@@ -161,6 +174,11 @@ interface SerializationContext {
      */
     fun withPreferredSerializationVersion(magic: SerializationMagic): SerializationContext
 
+    /**
+     * A shallow copy of this context but with the given (possibly null) encoding.
+     */
+    fun withEncoding(encoding: SerializationEncoding?): SerializationContext
+
     /**
      * The use case that we are serializing for, since it influences the implementations chosen.
      */
@@ -232,3 +250,8 @@ class SerializedBytes<T : Any>(bytes: ByteArray) : OpaqueBytes(bytes) {
 interface ClassWhitelist {
     fun hasListed(type: Class<*>): Boolean
 }
+
+@DoNotImplement
+interface EncodingWhitelist {
+    fun acceptEncoding(encoding: SerializationEncoding): Boolean
+}