Move bitswap to its own crate (#12242)

* Introduce `sc-network-bitswap`

Move the bitswap protocol out of `sc-network` to its own crate.

* Improve test coverage

* Remove mention of bitswap from `sc-network`

* Fix documentation

* Fix clippy and remove the void dependency

* Remove unneeded trait bound and bump prost to 0.11

substrate/client/network/bitswap/build.rs

@@ -0,0 +1,5 @@
+const PROTOS: &[&str] = &["bitswap.v1.2.0.proto"];
+
+fn main() {
+	prost_build::compile_protos(PROTOS, &["src/schema"]).unwrap();
+}