add ConsulDiscoverable typeclass and Scala 2 macro implementation

[bpholt]

Adding the ConsulDiscoverable[Alg] typeclass allows the macro-derived Uri, Uri.Authority, and Host values to be used with generic service types, e.g. in functions where the service is passed in and not a literal value.

I ran into this when trying to extract out some common code initializing Smithy4s clients from one of our internal codebases, so this should open up that possibility.

[bpholt]

I'm open to bikeshedding on the names here. This typeclass will have macro-instances for Smithy4s services annotated with the @discoverable trait. The host, uriAuthority, and uri values will be derived from the serviceName value on the @discoverable trait, e.g. uri will return consul://${serviceName}.

This will typically be used like this:

def smithyClient[F[_], Alg[_[_, _, _, _, _]]](client: Client[F],  // http4s client
                                              consulMiddleware: ConsulMiddleware[F], // middleware from this library
                                              service: Service[Alg], // generated by smithy4s
                                             )
                                             (implicit C: ConsulDiscoverable[service.Impl]): Resource[F, service.Impl[F]] =
  SimpleRestJsonBuilder.apply(service)
    .client(NatchezMiddleware.clientWithAttributes(client)(
      "rpc.system" -> "smithy",
      "peer.service" -> ConsulDiscoverable[service.Impl].host,
    ))
    .uri(ConsulDiscoverable[service.Impl].uri)
    .middleware(consulMiddleware)
    .resource

[bpholt]

And just to be clear, this does not work, because the macro can't resolve what service is at compile time:

def smithyClient[F[_], Alg[_[_, _, _, _, _]]](client: Client[F],  // http4s client
                                              consulMiddleware: ConsulMiddleware[F], // middleware from this library
                                              service: Service[Alg], // generated by smithy4s
                                             ): Resource[F, service.Impl[F]] =
  SimpleRestJsonBuilder.apply(service)
    .client(NatchezMiddleware.clientWithAttributes(client)(
      "rpc.system" -> "smithy",
      "peer.service" → HostFromService(service),
    ))
    .uri(UriFromService(service))
    .middleware(consulMiddleware)
    .resource

Implementing it as a typeclass does work, because there is a stable location (outside the function above) where the service is known and the ConsulDiscoverable instance can be generated and then passed implicitly into the function.

[bpholt]

My mental model of how macros work is that it's like the code block is copy/pasted into the spot where the macro is invoked. The imports, etc., that are in effect in that spot are what controls how types are resolved. So e.g. a prior version used the .some enhancement method from Cats (uriAuthority.some) instead of wrapping the value in an Option as below, which worked in places where the Cats syntax had been imported, but failed otherwise.

To avoid that kind of issue or other issues with ambiguous imports, I fully qualified everything in the quasiquote so that it should work regardless of context.