Prepare for publishing

[thekaveman]

• Add CODEOWNERS for @cal-itp/benefits-admin, require 1 approving review for merges to main
• Rename package to calitp-littlepay (in pyproject.toml)
• Version maintained as string in pyproject.toml; use CalVer (read by littlepay/__init__.py for use inside the codebase).
• Create GH Actions environment for release workflow
• Auto GH Release on tag