refactor: Split SafeBag serialization into its own function for PKCS#12 structures. (#12452)

* refactor: pkcs12's decode_encryption_algorithm now returns a type wrapping multiple return values

* refactor: moved pkcs12 safebag serialization into its own function called by pkcs12.serialize_key_and_certificates

* Added additional test for generating key-only pkcs12 files with encryption for coverage

* Restructure match statement in serialize_bags to be more readable

* PKCS12 passwords are now stored as PyBackedBytes again to avoid extra allocations

Author: crbednarz