Session password set/clear for password-protected PLCs

[gijzelaerr]

Problem

Sharp7 and rs-snap7 both support SetSessionPassword / ClearSessionPassword, allowing access to password-protected PLCs. python-snap7 only has get_protection() (read-only) — users cannot unlock PLCs that require a password.

Scope

• Add set_session_password(password) and clear_session_password() to snap7.Client
• These use the S7 protocol's USERDATA function group for password management
• Both sync and async clients

References

• Sharp7: SetSessionPassword / ClearSessionPassword
• rs-snap7: set_session_password / clear_session_password