Avoid opaque byte fields/containers (#35)

This PR deprecates the `bytes coroutine_state` field on `Poll{,Result}`
in favor of a `google.protobuf.Any` field. The additional type URL will
help systems introspecting the state; they would otherwise have to guess
what the opaque bytes represent.

This PR also adds a wrapper message for
[pickled](https://docs.python.org/3/library/pickle.html) Python values,
so that the Python SDK can attach a type URL to Python
[inputs](https://github.com/dispatchrun/dispatch-proto/blob/e74123286e7a74baa9f9e64321fceca89242ad04/dispatch/sdk/v1/call.proto#L34),
[outputs](https://github.com/dispatchrun/dispatch-proto/blob/e74123286e7a74baa9f9e64321fceca89242ad04/dispatch/sdk/v1/call.proto#L56)
and now `state`. Previously we were using the built-in
[`BytesValue`](https://github.com/protocolbuffers/protobuf/blob/4a5660c889ef9eb4161dded5c2e91c46f0c30924/src/google/protobuf/wrappers.proto#L120)
message, and so elsewhere had to guess what the opaque bytes represent.

buf.yaml

@@ -4,7 +4,9 @@ deps:
 - buf.build/bufbuild/protovalidate
 breaking:
   use:
-  - FILE
+  - WIRE_JSON
+  except:
+  - FIELD_SAME_ONEOF
 lint:
   use:
   - DEFAULT

dispatch/sdk/python/v1/pickled.proto

@@ -0,0 +1,10 @@
+syntax = "proto3";
+
+package dispatch.sdk.python.v1;
+
+// Pickled is a wrapper for a value that was serialized with
+// the Python pickle package.
+message Pickled {
+  // Pickled representation of a value.
+  bytes pickled_value = 1;
+}

dispatch/sdk/v1/poll.proto

@@ -5,13 +5,17 @@ package dispatch.sdk.v1;
 import "buf/validate/validate.proto";
 import "dispatch/sdk/v1/call.proto";
 import "dispatch/sdk/v1/error.proto";
+import "google/protobuf/any.proto";
 import "google/protobuf/duration.proto";
 
 // Poll is a directive to make asynchronous calls and join on their results.
 message Poll {
   // Snapshot of the coroutine state that will be used in the next run to
   // resume the function.
-  bytes coroutine_state = 1;
+  oneof state {
+    bytes coroutine_state = 1;
+    google.protobuf.Any typed_coroutine_state = 6;
+  }
 
   // Calls to make asynchronously.
   repeated Call calls = 2;
@@ -45,7 +49,10 @@ message Poll {
 message PollResult {
   // The coroutine state that was recorded by the coroutine when it was last
   // paused.
-  bytes coroutine_state = 1;
+  oneof state {
+    bytes coroutine_state = 1;
+    google.protobuf.Any typed_coroutine_state = 4;
+  }
 
   // The list of results from calls that were made by the coroutine and
   // completed during the poll.