Provide JSON file with valid TFMs, similar to what runtime.json provides for RIDs

In the context of a Jenkins plugin for .NET SDK support, it would be nice to be able to offer auto-completion for fields contain TFMs.

For RIDs this is already possible via the [RID Catalog](https://github.com/dotnet/runtime/blob/master/src/libraries/pkg/Microsoft.NETCore.Platforms/runtime.json).

However, for TFMs, there seems to be no equivalent. The CLI documentation refers to [this documentation page](https://docs.microsoft.com/en-us/dotnet/standard/frameworks) as the canonical reference.
That has a section that says
> The following table shows the target frameworks supported by the .NET Core SDK and the NuGet client.
While that strongly suggests an exhaustive list, it does not include a TFM the document mentions in its opening paragraphs, `Xamarin.iOS10`. It also does not mention `net5.0`, the current planned TFM for .NET 5.0.

So while I could try to extract the names from that document, it would not be a great improvement over just maintaining a list by hand.

It would be useful to have some sort of true list of valid TFMs, as exhaustive as possible, preferably in some machine-readable format (JSON preferred, XML fine too).