admin: Sign release artifacts (#4580)

Approach copied from OpenEXR. Let's try it!

Fixes #4404

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

.github/workflows/release-sign.yml

@@ -0,0 +1,67 @@
+# SPDX-License-Identifier: Apache-2.0
+# Copyright (c) Contributors to the OpenImageIO Project.
+
+#
+# Releases are signed via https://github.com/sigstore/sigstore-python.
+# See https://docs.sigstore.dev for information about sigstore.
+#
+# This action creates a .tar.gz of the complete OpenImageIO source tree at
+# the given release tag, signs it via sigstore, and uploads the
+# .tar.gz and the associated .tar.gz.sigstore credential bundle.
+#
+# To verify a downloaded release at a given tag:
+# 
+#   % pip install sigstore
+#   % sigstore verify github --cert-identity https://github.com/AcademySoftwareFoundation/OpenImageIO/.github/workflows/release-sign.yml@refs/tags/<tag> OpenImageIO-<tag>.tar.gz
+#
+
+name: Sign Release
+
+on:
+  release:
+    types: [published]
+
+permissions:
+  contents: read
+  
+jobs:
+  release:
+    name: Sign & upload release artifacts
+    runs-on: ubuntu-latest
+
+    env:
+       TAG: ${{ github.ref_name }}
+    permissions:
+      contents: write
+      id-token: write
+      repository-projects: write
+       
+    steps:
+
+      - name: Set Prefix
+        # The tag name begins with a 'v', e.g. "v3.0.3", but the prefix
+        # should omit the 'v', so the tarball "OpenImageIO-3.0.3.tar.gz"
+        # extracts files into "OpenImageIO-v3.0.3/...".  This matches
+        # the GitHub release page autogenerated artifact conventions.
+        run: |
+          echo OPENIMAGEIO_PREFIX=OpenImageIO-${TAG//v}/ >> $GITHUB_ENV
+          echo OPENIMAGEIO_TARBALL=OpenImageIO-${TAG//v}.tar.gz >> $GITHUB_ENV
+        shell: bash
+
+      - name: Checkout
+      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+
+      - name: Create archive
+        run: git archive --format=tar.gz -o ${OPENIMAGEIO_TARBALL} --prefix ${OPENIMAGEIO_PREFIX} ${TAG}
+
+      - name: Sign archive with Sigstore
+        uses: sigstore/gh-action-sigstore-python@f514d46b907ebcd5bedc05145c03b69c1edd8b46 # v3.0.0
+        with:
+          inputs: ${{ env.OPENIMAGEIO_TARBALL }}
+          upload-signing-artifacts: false
+          release-signing-artifacts: false
+
+      - name: Upload release archive
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: gh release upload ${TAG} ${OPENIMAGEIO_TARBALL} ${OPENIMAGEIO_TARBALL}.sigstore.json