Add integration branch for .net vNext, OData vNext, ASPNetCore vNext

[hippiehunter]

The high level goal is to be alerted anytime one of our three main dependencies has changed in a way that causes a break. The intention is that these alerts will be acted on in a timely manor to avoid getting behind. Some manual effort will be required when writing new features or fixes in the main harmony core branch.

• Add git submodule for OData git main branch
• Add git submodule for EFCore git main branch
• Update pipeline yaml to run nightly builds of .NET
• Configure pipeline runs to be nightly
• Add override mechanism for harmony core cli tool to point at the build artifacts (templates/nuget packages/source)