Feature: Adds support for version names for snapshots

[LordHepipud]

In order to easy manage different snapshot versions for different branches, we now support providing a name as version for snapshot packages only,

Example:

Add-IcingaRepository -Name 'icinga-powershell-framework/doc/doc_rewrite' -RemotePath 'https://packages.icinga.com/IcingaForWindows/snapshot/icinga-powershell-framework/doc/doc_rewrite/ifw.repo.json';
Install-IcingaComponent -Name 'framework' -Version 'doc/doc_rewrite' -Snapshot -Force;