Consume Optimization Training Data for CrossGen2 in build

[Pilchie]

We'll need to:

• update our build to use crossgen2
• update our build to consume optimization training data (in the form of NuGet packages) that the perf lab will be producing.

[Pilchie]

See dotnet/runtime#49793 for an example of some of this in dotnet/runtime

[davidwrighton]

The expectation is that the aspnet repo will be able to use the same packages as are used by the runtime repo, or that the runtime repo will produce a new set of packages for downstream consumption. Regardless, the priority is to enable crossgen2, and then enabling the pgo data should be relatively simple.

[Pilchie]

Would like to get this done no later than Preview 5, but not putting in that milestone in case we pick it up earlier.

Also tagging @dotnet/aspnet-build

[AndyAyersMS]

Is this done?

I see the crossgen2 update happened, and that it will consume PGO data if that data's available, but I can't tell if the data actually is available.

[dougbu]

Is this done?

Yes, @AndyAyersMS, complete with 46b56aa. Thanks for the reminder.

Thanks again to @davidwrighton for his #31778. My final changes (after lots of testing) wasn't much different than @davidwrighton's work.