api: generate field ownership functions

Introduce a `protoc` generator (`tools/protoc-gen-owners`) to
automatically generate the repetitive field ownership tracking functions.
This refactors the `pkg/api/owners.go` file.

The new generator uses the `Field` enum in `api.proto` as the source of
truth. The original `owners.go` file has been split logically into
generated and handwritten parts:
- `owners_generated.go`: Contains the auto-generated boilerplate for
  most fields.
- `owners.go`: Retains the core handwritten logic, including constructors,
  conflict resolution, and special-cased handlers for fields with custom
  logic (`OciHooks`, `Rdt`).

This change reduces manual boilerplate, improves maintainability, and ensures
consistency between the protobuf definitions and the Go API.

Assisted-by: gemini-cli
Signed-off-by: Samuel Karp <samuelkarp@google.com>

Author: samuelkarp

Makefile

@@ -80,12 +80,15 @@ FORCE:
 # build targets
 #
 
-build-proto: check-protoc install-ttrpc-plugin install-wasm-plugin install-protoc-dependencies build-protoc-gen-strip
+build-proto: check-protoc install-ttrpc-plugin install-wasm-plugin install-protoc-dependencies build-protoc-gen-strip build-protoc-gen-owners
 	for src in $(PROTO_SOURCES); do \
 		$(PROTO_COMPILE) \
 			--plugin=protoc-gen-strip=$(abspath $(BIN_PATH)/protoc-gen-strip) \
 			--strip_out=pkg/api \
 			--strip_opt=file=strip.go \
+			--plugin=protoc-gen-owners=$(abspath $(BIN_PATH)/protoc-gen-owners) \
+			--owners_out=pkg/api \
+			--owners_opt=file=owners_generated.go \
 			$$src; \
 	done
 	sed -i '1s;^;//go:build !wasip1\n\n;' pkg/api/api_ttrpc.pb.go
@@ -120,6 +123,11 @@ clean-cache:
 # plugins build targets
 #
 
+.PHONY: build-protoc-gen-owners
+build-protoc-gen-owners:
+	$(Q)echo "Building build/bin/protoc-gen-owners..."; \
+	$(GO_BUILD) -C tools/protoc-gen-owners -o $(abspath $(BIN_PATH)/protoc-gen-owners) $(GO_BUILD_FLAGS) .
+
 .PHONY: build-protoc-gen-strip
 build-protoc-gen-strip:
 	$(Q)echo "Building build/bin/protoc-gen-strip..."; \