feat: add capnp

Signed-off-by: kjuulh <contact@kjuulh.io>

crates/sq-capnp-interface/schema/data_plane.capnp

@@ -0,0 +1,65 @@
+@0xb8f6c1e2a3d4e5f6;
+
+struct MessageHeader {
+  key @0 :Text;
+  value @1 :Data;
+}
+
+struct PublishMessage {
+  topic @0 :Text;
+  key @1 :Data;
+  value @2 :Data;
+  headers @3 :List(MessageHeader);
+}
+
+struct PublishRequest {
+  messages @0 :List(PublishMessage);
+  ackMode @1 :UInt8;
+  producerId @2 :Text;
+}
+
+struct PublishResult {
+  topic @0 :Text;
+  partition @1 :UInt32;
+  offset @2 :UInt64;
+}
+
+struct PublishResponse {
+  results @0 :List(PublishResult);
+}
+
+struct SubscribeRequest {
+  topic @0 :Text;
+  partition @1 :UInt32;
+  consumerGroup @2 :Text;
+  startOffset @3 :UInt64;
+  hasStartOffset @4 :Bool;
+  maxBatchSize @5 :UInt32;
+}
+
+struct ConsumedMessage {
+  offset @0 :UInt64;
+  topic @1 :Text;
+  partition @2 :UInt32;
+  key @3 :Data;
+  value @4 :Data;
+  headers @5 :List(MessageHeader);
+  timestampMs @6 :UInt64;
+}
+
+struct SubscribeResponse {
+  messages @0 :List(ConsumedMessage);
+}
+
+struct AckRequest {
+  consumerGroup @0 :Text;
+  topic @1 :Text;
+  partition @2 :UInt32;
+  offset @3 :UInt64;
+}
+
+struct AckResponse {}
+
+struct ErrorResponse {
+  message @0 :Text;
+}