Add SystemDPAPIdump.py example

[Dramelac]

Original PR on fortra/impacket: fortra#1137

This PR adds a new example, SystemDPAPIdump.py, which automates extraction of DPAPI credentials for the SYSTEM user on a remote host.

Currently it extracts and decrypts DPAPI credentials files (used by the task scheduler, amongst others) and SCCM client credentials (following @gentilkiwi's publication and implementation), using a combination of WMI (SCCM), SMB client (retrieve files) and remote registry (retrieve DPAPI keys) features from the library.

Process:

• list and retrieve SCCM client secrets using WMI
• list and retrieve DPAPI credential files from the SYSTEM user profile using SMB
• parse retrieved credentials to obtain a list of masterkeys required for decryption
• retrieve required masterkey files using SMB
• dumps LSA Secrets to obtain dpapi_userkey (unless provided with -userkey, e.g. when previously obtained)
• cleanup (end of remote operations)
• decrypt masterkeys using dpapi_userkey
• decrypt credentials/secrets using masterkeys

Has some error management to attempt cleanup even if something goes wrong, and tries to not perform unneeded operations.
Reuses code from secretsdump.py, wmiquery.py, dpapi.py, and smbclient.py.