Add support for documenting routers

[aidansunbury]

This feature might be something only I am interested in, but I like the idea of being able to document information about the "object" that is grouping procedures.

The stripe api does this frequently. The swagger spec also supports tags having descriptions.

For an equivalent "pets" router, we should be able to specify that it is a router that is all about your pets.

Implementation

As far as I can tell, trpc does not support router metadata. The hacky way to do this would be to merge your base t.procedure with another router that just contains a dummy procedure to store the metadata.