Switch to using URLCredentialStorage to securely store tokens in the keychain

[Ast3r10n]

Tokens are currently stored in the UserDefaults dictionary, which is highly discouraged to store sensitive data such as authentication tokens.

Switch to storing tokens using URLCredentialStorage (which stores permanent credentials in the keychain) or the keychain itself to store tokens.

[hadiidbouk]

That's something I will definitely want to enhance, thank you for mentioning that!