Gateway: Add connection timeout, add Config to gateway. (#51)

This change fixes tasks hanging due to rare cases of messages being lost between full Discord reconnections by placing a configurable timeout on the `ConnectionInfo` responses. This is a companion fix to [serenity#1255](https://github.com/serenity-rs/serenity/pull/1255). To make this doable, `Config`s are now used by all versions of `Songbird`/`Call`, and relevant functions are  added to simplify setup with configuration. These are now non-exhaustive, correcting an earlier oversight. For future extensibility, this PR moves the return type of `join`/`join_gateway` into a custom future (no longer leaking flume's `RecvFut` type).

Additionally, this fixes the Makefile's feature sets for driver/gateway-only compilation.

This is a breaking change in:
* the return types of `join`/`join_gateway`
* moving `crate::driver::Config` -> `crate::Config`,
* `Config` and `JoinError` becoming `#[non_breaking]`.

This was tested via `cargo make ready`, and by testing `examples/serenity/voice_receive` with various timeout settings.

Cargo.toml

@@ -63,6 +63,10 @@ version = "0.3"
 optional = true
 version = "0.11"
 
+[dependencies.pin-project]
+optional = true
+version = "1"
+
 [dependencies.rand]
 optional = true
 version = "0.8"
@@ -142,11 +146,13 @@ default = [
 gateway = [
     "gateway-core",
     "tokio/sync",
+    "tokio/time",
 ]
 gateway-core = [
     "dashmap",
     "flume",
     "parking_lot",
+    "pin-project",
     "spinning_top",
 ]
 driver = [