Add compatibility for Powershell Core and make this snap-in a module

Powershell versions 5.1 and earlier are no longer in active development. Powershell Core (versions 6, 7, and the future) do not support snap-ins, so the ShareFile-PowerShell SDK is totally unusable.

This should be solvable by making it a module instead of a snap-in. Powershell Core is also based on .NET Core so there may be other considerations.

[Jhonrubia-LAS]

This is a must for the future of this SDK and seems that noone cares.
It will be easier to deploy on any enviroment.
Please make it real.

[lukeskrzypek-sf]

Please see https://github.com/sharefile-org/ShareFile-PowerShell-Module which is a port of this SnapIn to the Module format (supporting PowerShell 5.X and 7+)