More flexible interface for feature building

Features could allow arg/distribution combination based on a different variety of distributions and constructions.

## Idea

Introduce a `FeatureArgType` type so `build_features` can create more flexible types with a `build_args` function dispatching over `::FeatureArgType`
E.g. informally building the the classic scalar feature would involve:
```julia
abstract type FeatureArgType
Affine <: FeatureArgType

function build_args(
samp::sampler,
feature_parameters::Dict,
inputs::Array,
::Affine,
)
xi = get_distribution(samp)["xi"]
uniform = get_distribution(samp)["uniform"]
return inputs*xi + uniform
end
```
replacing the hardcoded https://github.com/CliMA/RandomFeatures.jl/blob/2d864cc8ffc89965b476bb04db1f2f6d687c881b/src/Features.jl#L148-L156
- Possibly move the move the inbuilt uniform distribution elsewhere? It's only in the fourier case that this is necessary