Use the `github.com/grpc-ecosystem/grpc-gateway` project to
automatically generate REST handlers for portions of the Journals
and Shards gRPC services.

Also implement a CORS wrapper with configurable allowed origins.
Together these changes allow Gazette brokers and consumers to be
directly queried by browsers in cross-origin contexts.

Note that REST handlers automatically forward `Authorization: Bearer`
tokens into their corresponding gRPC handlers, so authorization checks
"just work".

Allow consumers applications to provide a function which wraps around a
raw net.Listener and a loaded *tls.Config, returning another
net.Listener, in order to customize the manner in which net.Conn
instances are accepted and built.