Make S3Path a concrete type, with the config as a field only not a type param

[nickrobinson251]

the type param was added in #140

but this change makes S3Path non-concrete, which in practice makes working with S3Paths much more painful
e.g. one has to dispatch on Vector{<:S3Path} rather Vectpr{S3Path} (so was breaking), and can also lead to methods being compiled (specialised) on the Config type which is usually of no importance. instead, the Config could just be a field, which we can check when necessary, and in this context dynamic dispatch is probably fine (#140 (comment))

in my experience of maintaining a (private) codebase using S3Paths, the addition of th type parameter has been a continuing source of pain. since the same flexibility can be supported without the Config being stored at the type level, i suggest we remove it (in the next breaking release)