Create install-prereq command

This would consolidate the various third party installer code into a single command (and make adding new ones less work.) For now, the only two things included would be the MSSQL Authentication dll and the LLVM installer, but I am planning to add more down the road. This should use ClassDiscovery to allow extensions to take advantage of these facilities as well. We should also support multiple versions and support for detecting existing installations (with option to force install anyways). There would also be support for post install execution as well as custom download locations.