`Flagsmith/setup-cli` GitHub action

Make the following job work:

```yaml
jobs:
  test-flagsmith-auth:
    runs-on: ubuntu-latest

    permissions:
      id-token: write
      contents: read

  steps:
     - uses: Flagsmith/setup-cli@v1
     - run: flagsmith auth status   # prove the CLI is installed, and we're logged in
```

Support an optional `api-url` input.
