feat: extract kafkamgmt.v1 proto contract from kafaka_management_service

Single source of truth for the wire contract shared by the server,
kmsvc-sdk, and kmsvc-cli. Ships proto source + buf lint/breaking-change
config only; consumers submodule this repo and codegen independently
with their own go_package override (see README.md).

buf.yaml

@@ -0,0 +1,11 @@
+version: v2
+modules:
+  - path: proto
+deps:
+  - buf.build/googleapis/googleapis
+lint:
+  use:
+    - STANDARD
+breaking:
+  use:
+    - FILE