feat(auth): implement plaintext, hashed, and api key authentication methods

[MattiaPun]

This implements the reworked authentication system. Users now have the option to choose between plaintext, hashed (token + salt), and api_key methods to authenticate.

The configuration file will auto-migrate as usual. This is the new layout of the credentials.toml file:

[server]
url = ""
username = ""
auth_method = "plaintext" # Options: "plaintext", "hashed", "api_key"

# Required if auth_method = "plaintext"
password = ""

# Required if auth_method = "hashed"
password_hash = ""
password_salt = ""

# Required if auth_method = "api_key"
api_key = ""

Important Notes:

• Username requirement for API keys: You still have to provide a username when using an API key. Even though it is not strictly needed during API calls, we use the /getUser endpoint on startup and login to validate the credentials, which requires the username.
• API Key implementation: The api_key method is currently a theoretical implementation. I cannot actually test it against my own backend yet, but it has been built to be strictly conformant with the Subsonic API specification.

Resolves #41