feat(e2ee): support password override for key deletion

[marinofaggiana]

Summary

Add support for supplying an optional account password when deleting End-to-End Encryption keys.

The password is forwarded to the authorization header for:

• Public key and certificate deletion
• Private key deletion
• Complete E2EE key deletion

When the supplied password is nil or empty, NextcloudKit continues using the password stored in the current session, preserving the existing behavior.

Motivation

Some E2EE key-management operations may require the user’s account password instead of the password or token stored in the active session.

Allowing callers to provide an explicit password makes these operations work with servers requiring password confirmation without changing the authentication behavior of existing callers.

Testing

• Verified public key deletion with an explicitly supplied password.
• Verified private key deletion with an explicitly supplied password.
• Verified complete E2EE key deletion with an explicitly supplied password.
• Verified that nil or empty passwords fall back to the current session credentials.
• Verified that the existing request-header behavior remains unchanged for other API calls.