Add protobuf generated code for Go

.gitattributes

@@ -0,0 +1 @@
+*.pb.go linguist-generated

.github/workflows/generate.yml

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+#
+# Copyright 2022 The Sigstore Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: Generate code
+
+permissions:
+  contents: read
+
+on:
+  push:
+    paths: 
+      - '**.proto'
+  pull_request:
+    paths: 
+      - '**.proto'
+
+jobs:
+  generate_go:
+    name: Generate go code
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8 # v3
+
+    - name: Compile protobuf
+      run: |
+        make proto
+
+    - name: Ensure no files were modified as a result of the codegen
+      run: git update-index --refresh && git diff-index --quiet HEAD -- || git diff --exit-code

Makefile

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+#
+# Copyright 2022 The Sigstore Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+PROTOC_IMAGE := namely/protoc-all@sha256:07f1ba9dbe11f5675e2efc8617c9552217dc4c3eb5ccd108f7c3889878dbae50 # v1.50_0
+
+# generate Go protobuf code
+proto: 
+	@echo "Generating Protobuf files"
+	docker run --platform linux/amd64 -v ${PWD}:/defs ${PROTOC_IMAGE} -i protos -f envelope.proto -f sigstore_bundle.proto -f sigstore_common.proto -f sigstore_rekor.proto -l go --go-module-prefix github.com/sigstore/protobuf-specs/gen/pb-go

README.md

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 # protobuf-specs
+
+This repository holds protobuf specifications for Sigstore messages.
+
+## Protobuf
+
+If you change protobuf definitions, you will need to regenerate the code by running the protocol buffer compiler on the changed `.proto` files.
+
+You will need [Docker](https://docs.docker.com/get-docker/) installed to generate the protobuf stubs. Then run, 
+
+```
+$ make proto
+```
+
+to generate the `*.pb.go` files under `gen/`.