Move DCT functions into separate namespace

[rurounijones]

Move the DCT functions out of the custom namespace and into a dct namespace. All future Custom, framework specific, work will be done in an isolated namespace.

As part of this we will rename the lua methods with a prefix as suggested by @justin-lovell to help with isolation and reduce chances of conflict. (i.e. dct.joinMission). We may eventually rename all existing core methods as well but that is out of scope for this.

[justin-lovell]

FYI, I saw @rkusa make some progress with this element - https://github.com/DCS-gRPC/rust-server/tree/poc-plugins

He is smarter than me, but I see that as a way to have "experimental API's" or "beta API's" sandboxed and eventually promoted. This would give us some sandbox to attempt to building the group unit.