fix: add rules_proto and explicit load statements for proto_library / cc_proto_library

[fcostaoliveira]

Problem

CI fails on both build-and-test and test-deb-build jobs with:

ERROR: src/BUILD:169:1: name 'proto_library' is not defined
ERROR: src/BUILD:174:1: name 'cc_proto_library' is not defined
ERROR: src/quipper/BUILD:23:1: name 'proto_library' is not defined
ERROR: src/quipper/BUILD:29:1: name 'cc_proto_library' is not defined
...

Root cause

Bazel 9.1.0 (used by CI) with Bzlmod no longer auto-loads proto_library and cc_proto_library as built-in rules. These symbols must now be explicitly imported via load() statements, and rules_proto must be declared as an explicit dependency in MODULE.bazel.

Fix

1. MODULE.bazel — Add bazel_dep(name = "rules_proto", version = "7.1.0").

2. src/BUILD — Add at the top:

   load("@rules_proto//proto:defs.bzl", "proto_library")
   load("@com_google_protobuf//bazel:cc_proto_library.bzl", "cc_proto_library")

3. src/quipper/BUILD — Same two load statements at the top.

rules_proto 7.1.0 is the current stable release in the Bazel Central Registry and is compatible with Bazel 9.x and protobuf 31.x+.