Make ProcessInfo / DiagnosticClient.GetProcessInfo public

In the same principle than #5086, I'm trying to chase down internal APIs that require to be accessed via `System.Reflection` or that are used also outside of this repository by other .NET tooling.

I would like to make `ProcessInfo` and `DiagnosticClient.GetProcessInfo` public. It seems that the API has been fairly stable in the past years with what looks like mostly additions to the API. This API is particularly interesting to surface target process information (runtime version, RID...etc.)

I can make a PR if this proposal is accepted.