add sample github action for crates.io publishing

Add a github action sample workflow file for auto-publishing crates on
creation of 'v*' tags. The sample workflow assumes a non-workspace
setup, so that cargo publish can be run from the repository root (=
crate root).

For multi-crate repositories, repository_setup.sh will generate
derivatives of this file.

There is a useless `cd .` in the `run` step which only exists so that
sed can easily replace it with the path for crates in a workspace when
auto-generating workflows.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

publish.yml

@@ -0,0 +1,21 @@
+name: Publish to crates.io
+
+on:
+  push:
+    tags: ['v*']  # Triggers when pushing version tags
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    # environment: release  # Optional: for enhanced security
+    permissions:
+      id-token: write     # Required for OIDC token exchange
+    steps:
+    - uses: actions/checkout@v4
+    - uses: rust-lang/crates-io-auth-action@v1
+      id: auth
+    - run: |
+        cd .
+        cargo publish
+      env:
+        CARGO_REGISTRY_TOKEN: ${{ steps.auth.outputs.token }}