- feat(action_type.py): add ContractCreation action type
- feat(base.py): add base Action class and EventHandleFunc type
- feat(tx_type.py): update actions field in TaggedTx to accept List[Action]

… up to date before downloading

- chore(installer.py): add support for saving checksum of downloaded file

- feat(decodex): add support for downloading tags and signatures for Ethereum chain
- Add installer module with download_github_file function
- Add constant module with DECODEX_DIR constant
- Add download_github_file function to download tags and signatures for Ethereum chain
- Add callbacks module with BaseBeforeCallback, BasePostCallback, GithubRawBeforeCallback, GithubLFSBeforeCallback, GzipPostCallback, and ChecksumPostCallback classes
- Add utils module with get_github_url, get_github_api_url, and get_github_rawfile_url functions
- Update __init__.py to import installer module and constant module
- Update __main__.py to import download_github_file function from decodex.installer module
- Update cli function in __main__.py to add download command with chain argument and verify-ssl option
- Add clean command to remove downloaded tags and signatures for a chain

Feature/checksum