[user version] support user release version on deploy

[gigatim]

Summary

Add support for a release_version input to the gigalixir-action so users can pass a version number that gets injected into their Elixir app at build time.

Changes

1. Add optional release_version input to action.yml
2. When set, pass it as an HTTP header during git push: git -c http.extraHeader="X-Gigalixir-Release-Version: <version>" push gigalixir main
3. Document the new input in the action's README
4. Add an example workflow showing integration with a semantic release action (e.g., cycjimmy/semantic-release-action) where the resolved version is passed to the deploy step

Example usage

- uses: gigalixir/gigalixir-action@master
  with:
    release_version: ${{ steps.semantic.outputs.new_release_version }}

Notes

• Input is optional — existing workflows are unaffected
• The build server reads the header and exports GIGALIXIR__USER_RELEASE_VERSION for the buildpack
• User must opt in on their side by adding version: System.get_env("GIGALIXIR__USER_RELEASE_VERSION") || "0.0.1" to mix.exs

Parent: gigalixir/build-server#13