Feat: create/import keystore wallet (password-encrypted) + docstrings

[philogicae]

• Add keystore wallet compatibility: Encrypt the private key with a password and store it as a JSON file.
• When private key file is missing, prompt user to either:
  1. Create a new private key
  2. Import an existing one
  3. Import it from a passphrase.
• lru_cache for the last loaded private key (to avoid reentering password too often)

[github-actions]

"summary": "PR introduces new key management functions and refactors existing code for better security and usability.",
"details": [
"The PR introduces two new functions, create_key and save_key, which provide a more user-friendly way to manage private keys.",
"create_key allows the user to either import an existing private key or generate a new one.",
"save_key saves the private key to a file, supporting both unencrypted .key files and encrypted .json keystore files.",
"The existing get_fallback_private_key function has been refactored to use the new load_key function, which supports loading keys from both unencrypted and encrypted files.",
"The PR also includes changes to the configuration settings to support the new file formats for private keys.",
"The refactoring aims to improve the security and usability of the private key management in the SDK."
],
"highlight": [
"def create_key() -> bytes:",
"def save_key(private_key: bytes, path: Path):",
"def load_key(private_key_path: Path) -> bytes:",
"settings.PRIVATE_KEY_FILE = Path(settings.CONFIG_HOME, "private-keys", pk_file)"
]
}

[hoh]

Interesting feature to add.

I added some comments regarding the structure of the code, as a clearer separation of concert would make the code easier to test and reuse by not prompting the user within "deep" functions.

I am confused regarding the terms of password, passphrase and mnemonic in these changes.
Still regarding the use of passphrases and passwords, can thos be of length 64 or start with 0x ?

These new features have no test coverage yet, would it be better to mark the Pull Request as Draft while waiting for those to be implemented ?

[hoh]

Users should be prompted in the CLI, not in the SDK.

[hoh]

Why or "pytest" in sys.modules ?

[hoh]

Users should be prompted in the CLI, not in the SDK.

[hoh]

Why this mention of pytest here ?

[hoh]

I will update my comments about the prompts, I forgot that this branch is regarding the SDK and user interactions should go in the CLI aleph-client.