refactor(provider): prefer v2 JsonRpcServer; wrap legacy engines; tighten v2 types

- InternalProvider
  - replace rpcHandler with server
  - wrap legacy engine via asV2Middleware + JsonRpcServer
  - update tests
- asV2Middleware: fix middleware signature
- providerFromMiddleware
  - Use providerFromMiddlewareV2 and asV2Middleware to wrap legacy
    middleware
- json-rpc-engine:
  - add MiddlewareConstraint
  - update JsonRpcServer/asV2Middleware generics

Author: rekmarks