samba-tool: add 'user getpassword' command

samba-tool: add 'user getpassword' command

This provides an easy way to get the passwords of a user
including the cleartext passwords (if stored) and derived
hashes. This is done by providing virtual attributes like:
virtualClearTextUTF16, virtualClearTextUTF8,
virtualCryptSHA256, virtualCryptSHA512, virtualSSHA

This is much easier than using ldbsearch and manually parsing
the supplementalCredentials attribute.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>