Enable selection of latest SDK

[idg10]

Because of #22 we currently target .NET Core SDK 3.1.102.

Even though the SpecFlow issue that drove this SpecFlowOSS/SpecFlow#1912 is marked as closed, at the time of writing, this fix is still only in preview builds of SpecFlow. (It's fixed in 3.3.)

However, it's hard to test whether we still have this problem without modifying these templates, at which point we risk breaking everything again.

So it seems to make sense to introduce some sort of option for controlling the SDK version.

In the long run, we want to put the default SDK back to 3.x, so it would be useful to have a way to opt in to using a specific version when necessary. For now, we want a way to opt into using 3.x so we can test ahead of imposing it on everyone