Replication target dataset encryption

New replication task API allows to encrypt target dataset:

Bool( "encryption" , default = False ),
Str( "encryption_key" , null = True , default = None ),
Str( "encryption_key_format" , enum =[ "HEX" , "PASSPHRASE" ], null = True , default = None ),
Str( "encryption_key_location" , null = True , default = None ),

This should behave the same way as when creating local dataset (user is allowed to enter key/passphrase or generate and download key). All fields are non-null when target dataset encryption is enabled.

encryption_key_location is a remote file system path for the key. It also has a special value $TrueNAS which indicates that key will be stored in TrueNAS >= 12.0 database (this should probably be played as checkbox + text field).

We need to integrate those changes into both wizard and classic form.