feat: Define a router independently of its implementation

Problem

Describe the feature you'd like to request See example repo (https://github.com/rhinodavid/trpc-server-definition) for all the details My request is similar to the one @iduuck wrote about in https://github.com/trpc/trpc/issues/3496#issuecomment-1425705704. tldr: One should not need the server implementation in order to get a typed client. Currently this is not possible (I don't think -- I"m pretty new here) -- all the examples I've seen create the client using the `typeof` the AppRouter definition. Describe the solution you'd like to see My proposed solution is to create utility types to define the router type. Usage would look like this: [code block] In the server implementation, you'd enforce that the server satisfies the interface: [code block] Then the client is typed with the interface: [code block] The example repo has a complete(?) working PoC. Describe alternate solutions Another possible solution is a codegen step as described in https://github.com/trpc/trpc/issues/3496#issuecomment-1425705704. The pro of this solution is it keeps the existing pattern of [implement router] -> [extract interface]. Some might prefer this, but after a long time using gRPC/stubby I'm more used to the [define interface] -> [implement router] pattern. This pattern is better for large organizations where multiple teams might need to debate and agree on the interface. The con of the solution is codegen can be finicky. It also doesn't easily solve the workspace organization probl