Base64 decode image attribute

[Kinsteen]

Piggy backing on #154.

Users are complaining (#163) because the LDAP attribute expect the binary data of the JPG picture, but for some LDAP servers (Authentik for example) we can't store binary data as it breaks serialization. Instead, this plugin will decode from Base64 to get the binary data. The best way could be to have like a drop-down to select "Binary", "Base64" and even "URL" (but I'm not versed enough in C# for this one), but I've tested this way and it works.