This removes custom content-types in favor of
just using `application/grpc`. There is some
confusion around the specification but most
grpc implementations ignore the `+` and
anything after.

* feat(transport): Add service multiplexing/routing

This change introduces a new "router" built on top of
`transport::Server` that allows one to run multiple
gRPC services on the same socket.

```rust
Server::builder()
    .add_service(greeter)
    .add_service(echo)
    .serve(addr)
    .await?;
```

There is also a new `multiplex` example showcasing
server side service multiplexing and client side
service multiplexing.

BREAKING CHANGES: `Server::serve` is now crate private
and all services must be added via `Server::add_service`.
Codegen also returns just a `Service` now instead of a
`MakeService` pair.

Closes #29

Signed-off-by: Lucio Franco luciofranco14@gmail.com

Closes #81

* chore: Add `cargo-hack` to check features

Signed-off-by: Lucio Franco <luciofranco14@gmail.com>

* Update .github/workflows/CI.yml

Co-Authored-By: Taiki Endo <te316e89@gmail.com>

This makes it so you can check if the initial connection
is established. Before this we used reconnect which would
lazily attempt to connect. So if you were trying to connect
to a non existant Server you wouldn't find out until after
you attempted your first RPC. This simplifies everything
by allowing you connect before creating the RPC client.

BREAKING CHANGE: `Endpoint::channel` was removed in favor of
an async `Endpoint::connect`.

Signed-off-by: Lucio Franco <luciofranco14@gmail.com>