feat: replace ProtocolName with AsRef<str>

Previously, a protocol could be any sequence of bytes as long as it started with `/`. Now, we directly parse a protocol as `String` which enforces it to be valid UTF8.

To notify users of this change, we delete the `ProtocolName` trait. The new requirement is that users need to provide a type that implements `AsRef<str>`.

We also add a `StreamProtocol` newtype in `libp2p-swarm` which provides an easy way for users to ensure their protocol strings are compliant. The newtype enforces that protocol strings start with `/`. `StreamProtocol` also implements `AsRef<str>`, meaning users can directly use it in their upgrades.

`multistream-select` by itself only changes marginally with this patch. The only thing we enforce in the type-system is that protocols must implement `AsRef<str>`.

Resolves: #2831.

Pull-Request: #3746.

protocols/gossipsub/src/handler.rs

@@ -79,6 +79,7 @@ pub enum HandlerIn {
 /// creation loops.
 const MAX_SUBSTREAM_ATTEMPTS: usize = 5;
 
+#[allow(clippy::large_enum_variant)]
 pub enum Handler {
     Enabled(EnabledHandler),
     Disabled(DisabledHandler),