fix typescript types for fetcher

[itajaja]

fetcher shouldn't really return a SigningKey, becuase that's a complex instance with methods. there might be a better signature than any but I think this is good enough considering the scope of this (ie we don't want to define all possible fields of a jwk)

reopened from #230

[meeq]

Consider using the unknown type instead:

unknown is the type-safe counterpart of any. Anything is assignable to unknown, but unknown isn’t assignable to anything but itself and any without a type assertion or a control flow based narrowing. Likewise, no operations are permitted on an unknown without first asserting or narrowing to a more specific type.

[itajaja]

@meeq you are totally right, unknown would have been the right type. since fetcher is a function that as a client you are not suppose to use, but just to set, the "damage" of having it as any is somewhat reduced, because you are not supposed to call the fetcher method in the first place