Accept a function as `gatsbyTypePrefix` .

[AbdallahAbis]

I'm working with Sanity Graphql, some remote schema types use TypeName and others use SanityTypeName, how can I achieve all generated Gatsby types say SanityRemoteTypeName, providing a gatsbyTypePrefix = "Sanity" will cause some types to become SanitySanityRemoteTypeName and not providing any gatsbyTypePrefix will result in some fields being RemoteTypeName.

So I think gatsbyTypePrefix option should accept a function that will get passed the RemoteTypeName and it will check if it's already prefixed with Sanity then do nothing otherwise prefix with Sanity