Add introspection support to backend

[rafalp]

tRPC has no schema or introspection capability, but we could let devs opt in to one.

Introspection query

Backend class could accept introspection option that if set, would add introspection query to the server, eg:

trpc_backend = Backend(..., introspection="introspection")

Running "introspection" against the server would then return a JSON (in unknown format) with list of public mutations/queries and types

Introspection method

Backend class could have introspection() method returning datastruct with information about possible mutations, queries and their types. Out of the box there wouldn't be anything else going on with this structure, but it would be useful for tooling developers to write codegens or custom "API contents" page.