Change the namespace of the public "InferenceMode" enum

[mlaily]

This is a breaking change, but I feel like it's needed...

Using FSharp.Data.Runtime.StructuralInference was a mistake (sorry, my bad). It should not be necessary to reference that namespace just to initialize a type provider...

I moved the enum to the FSharp.Data namespace instead, the same as the providers so it's readily available:

• Nothing has to be explicitly opened to use the enum with a type provider.
• This will hopefully make the breaking change less painful since it is likely to still compile after the update.

I have no idea how much this new-ish enum is used in the wild, but since it's optional and a bit obscure, it might not impact too many users...

Since the next version will be the new major version 6.x and it's still currently in beta, I think this is a good opportunity to do this kind of thing now.

[cartermp]

Definitely fine with the change here.