Add CountBy and AggregateBy benchmarks

[manandre]

Adding benchmarks for the new CountBy and AggregateBy Linq methods.
See dotnet/runtime#91507 and dotnet/runtime#92089

I have tried to add support for the net9.0 TFM as it is now the one used in the main branch of the dotnet/runtime repo but I don't know what I'm getting into...

[manandre]

/datadisks/disk1/work/B3FD0979/w/BB2809A4/e/tools/dotnet/x64/sdk/9.0.100-alpha.1.23464.17/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(166,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 9.0.  Either target .NET 8.0 or lower, or use a version of the .NET SDK that supports .NET 9.0.

Why does the latest nightly build of the .NET SDK (9.0.100-alpha.1.23464.17) does not support targeting .NET 9.0?!?

[manandre]

We will have to wait...
The latest nightly dotnet SDK still does not support the net9.0 TFM.
See dotnet/sdk#35031

[cincuranet]

All the changes to net9.0 and around need to happen in multiple places across multiple repositories to keep our measurements and reporting correct. We'll be doing that shortly. I suggest to you to keep only changes/additions to benchmarks in this PR. And I'll go back to this when all net9.0 pieces are in place. It should fit in without any friction.

[manandre]

Agreed. I have reworked the commits to only contain benchmarks.

[cincuranet]

Thanks. And excuse the longer waiting. I think this is ready for merging now.