Add Codec import in macro generated code

Author: aki-ks

bson/src/main/scala/org/mongodb/scala/bson/codecs/macrocodecs/CaseClassCodec.scala

@@ -311,7 +311,7 @@ private[codecs] object CaseClassCodec {
  q"""
  import scala.collection.mutable
  import org.bson.{ BsonInvalidOperationException, BsonWriter }
- import org.bson.codecs.EncoderContext
+ import org.bson.codecs.{ EncoderContext, Codec }
  import org.bson.codecs.configuration.CodecRegistry
  import org.mongodb.scala.bson.codecs.macrocodecs.MacroCodec