Add command to get an attachment (#1080)

* Add command to get an attachment

* Refactor retrieving of attachments to use StreamDetails

* Refactor AttachmentCommand to GetAttachmentCommand

* Minor improvements to GetAttachmentCommand

* Use JSON serializer to serialize binary data

Serializing the stream is better for memory handling than
loading the whole thing into the file.

* Clean up unneeded class

* Added command to doc

Co-authored-by: cedb <cedb@keylimebox.org>

src/main/java/org/asamk/signal/json/JsonAttachmentData.java

@@ -0,0 +1,9 @@
+package org.asamk.signal.json;
+
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
+
+import java.io.InputStream;
+
+public record JsonAttachmentData(
+        @JsonSerialize(using=JsonStreamSerializer.class) InputStream data
+) {}

src/main/java/org/asamk/signal/json/JsonStreamSerializer.java

@@ -0,0 +1,20 @@
+package org.asamk.signal.json;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.JsonSerializer;
+import com.fasterxml.jackson.databind.SerializerProvider;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public class JsonStreamSerializer extends JsonSerializer<InputStream> {
+
+    @Override
+    public void serialize(
+            final InputStream value,
+            final JsonGenerator jsonGenerator,
+            final SerializerProvider serializers
+    ) throws IOException {
+        jsonGenerator.writeBinary(value, -1);
+    }
+}