Helix: make the aspnetcore shared framework available in Helix tests

[natemcmaster]

Required to unblock tests such as #8270

Tests which need to run against the ASP.NET Core shared framework currently can't run in Helix. We would like to make more tests actually run on the shared framework (#4257 ), but we need to solve this issue first.

Challenges

• The shared framework is platform-specific, but Helix tests only build on Windows x64.
• Need to match the right shared framework runtime identifier with each Helix queue
• We don't currently include the .NET Core SDK or runtimes in the helix test payload because its inefficient to upload large payloads. Because we want to test the shared framework we just built, we will need to upload it in the test payload. Hopefully this doesn't hurt perf too much

Ideas to consider

• We already build the shared framework for each RID. It would probably be good to make Helix testing run as a part of our normal build and merge build and test.
• Might be worth investigating how corefx does this. For example, checkout https://github.com/dotnet/arcade/tree/master/src/Microsoft.DotNet.Build.Tasks.Configuration - they've expanded their use configurations to support concepts like 'AnyOS' along with 'AnyCPU'

cc @HaoK @ryanbrandenburg @aspnet/build

[Pilchie]

@dougbu @HaoK Is this still something we're targeting for 3.0?

[HaoK]

I believe we agreed that further helix work would be done post 3.0

[HaoK]

Initial support of this was added via #19177

It only puts the x64 shared framework for now.

[HaoK]

Still need to add support for building/matching appropriate RIDs to helix queues which is a bit more involved. Currently template tests are the only tests using this support