core/: Introduce rsa feature flag to avoid ring dependency (#2860)

- Introduce `rsa` feature flag to `libp2p-core`.
- Expose `rsa` feature in `libp2p`.
- Add `rsa` feature to `libp2p` `default`.

protocols/gossipsub/src/protocol.rs

@@ -613,6 +613,7 @@ mod tests {
     struct TestKeypair(Keypair);
 
     impl Arbitrary for TestKeypair {
+        #[cfg(feature = "rsa")]
         fn arbitrary<G: Gen>(g: &mut G) -> Self {
             let keypair = if g.gen() {
                 // Small enough to be inlined.
@@ -624,6 +625,12 @@ mod tests {
             };
             TestKeypair(keypair)
         }
+
+        #[cfg(not(feature = "rsa"))]
+        fn arbitrary<G: Gen>(_g: &mut G) -> Self {
+            // Small enough to be inlined.
+            TestKeypair(Keypair::generate_ed25519())
+        }
     }
 
     impl std::fmt::Debug for TestKeypair {