Remove PowerFxConfig.AdditionalFunctions

[MikeStall]

Remove PowerFxConfig.AdditionalFunctions

Power-Fx/src/libraries/Microsoft.PowerFx.Core/Public/Config/PowerFxConfig.cs

Line 55 in 0fa1968

internal readonly Dictionary<TexlFunction, IAsyncTexlFunction> AdditionalFunctions = new ();

This is a layering violation. It's a map in Config that maps TexlFunction to Interpreter implementations.
But Config lives in Fx.Core, so it can't point to interpreter implementations.

Having "EnableXXX" on config that adds function implementations forces a problem here.

One workaround we used elsewhere was like for Json - add a derived TexlFunction that implements the interpreter interface.