feat: add configs input for setting env vars

[gigatim]

Summary

• Adds a configs input that accepts multiline KEY=VALUE pairs for setting Gigalixir environment variables before deploy
• This avoids the GitHub Actions warnings caused by undeclared config_* dynamic inputs
• The legacy config_* prefix approach still works for backwards compatibility
• The create action now also applies configs (previously only deploy and create_deploy did)
• When used with copy_config_from, the copy runs first, then explicit configs are applied on top
• Updated README examples to use the new configs input

Usage

- uses: gigalixir/gigalixir-action@v1
  with:
    gigalixir_email: ${{ secrets.GIGALIXIR_EMAIL }}
    gigalixir_api_key: ${{ secrets.GIGALIXIR_API_KEY }}
    app_name: my-app
    configs: |
      MIX_ENV=prod
      SECRET_KEY_BASE=${{ secrets.SECRET_KEY_BASE }}
      DATABASE_POOL_SIZE=10

Closes gigalixir/management#25

Test plan

• All 43 tests pass (make all)
• Tests for multiline KEY=VALUE parsing
• Tests for values containing = signs (e.g., database URLs)
• Tests for blank lines and comments being skipped
• Tests for warnings on invalid lines
• Tests for merging new configs input with legacy config_* prefix