feat: automate publishing releases to Bazel Central Registry

.bcr/README.md

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+# Bazel Central Registry
+
+When protobuf is released, we want it to be published to the
+Bazel Central Registry automatically:
+<https://registry.bazel.build>
+
+This folder contains configuration files to automate the publish step.
+See <https://github.com/bazel-contrib/publish-to-bcr/blob/main/templates/README.md>
+for authoritative documentation about these files.

.bcr/metadata.template.json

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+{
+  "homepage": "https://github.com/protocolbuffers/protobuf",
+  "maintainers": [
+    {
+      "email": "sandyzhang@google.com",
+      "github": "zhangskz",
+      "name": "Sandy Zhang"
+    },
+    {
+      "email": "mkruskal@google.com",
+      "github": "mkruskal-google",
+      "name": "Mike Kruskal"
+    },
+    {
+      "email": "gberg@google.com",
+      "github": "googleberg",
+      "name": "Jerry Berg"
+    }
+  ],
+  "repository": ["github:protocolbuffers/protobuf"],
+  "versions": [],
+  "yanked_versions": {}
+}

.bcr/presubmit.yml

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+matrix:
+  platform: ["debian10", "macos", "ubuntu2004", "windows"]
+
+tasks:
+  verify_targets:
+    name: "Verify build targets"
+    platform: ${{ platform }}
+    build_targets:
+    - '@protobuf//:protobuf'
+    - '@protobuf//:protobuf_lite'
+    - '@protobuf//:protoc'
+    - '@protobuf//:test_messages_proto2_cc_proto'
+    - '@protobuf//:test_messages_proto3_cc_proto'
+
+bcr_test_module:
+  module_path: "examples"
+  matrix:
+
+    platform: ["debian10", "macos", "ubuntu2004", "windows"]
+  tasks:
+    run_test_module:
+      name: "Run test module"
+      platform: ${{ platform }}
+      build_targets:
+      - "//..."

.bcr/source.template.json

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+{
+  "integrity": "**leave this alone**",
+  "strip_prefix": "{REPO}-{VERSION}",
+  "url": "https://github.com/{OWNER}/{REPO}/releases/download/{TAG}/{REPO}-{VERSION}.zip"
+}