Rename getKey -> getPublicKey (#621)

* rename getKey to getPublicKey

* use publicKey directly in gossipsub

* update error messages

libp2p/protocols/secure/noise.nim

@@ -602,11 +602,9 @@ proc new*(
   outgoing: bool = true,
   commonPrologue: seq[byte] = @[]): T =
 
-  let pkBytes = privateKey
-  .getKey()
+  let pkBytes = privateKey.getPublicKey()
   .expect("Expected valid Private Key")
-  .getBytes()
-  .expect("Couldn't get Private Key bytes")
+  .getBytes().expect("Couldn't get public Key bytes")
 
   var noise = Noise(
     rng: rng,

libp2p/protocols/secure/secio.nim

@@ -435,16 +435,14 @@ proc new*(
   T: typedesc[Secio],
   rng: ref BrHmacDrbgContext,
   localPrivateKey: PrivateKey): T =
-  let pkRes = localPrivateKey.getKey()
+  let pkRes = localPrivateKey.getPublicKey()
   if pkRes.isErr:
-    raise newException(Defect, "Can't fetch local private key")
+    raise newException(Defect, "Invalid private key")
 
   let secio = Secio(
     rng: rng,
     localPrivateKey: localPrivateKey,
-    localPublicKey: localPrivateKey
-      .getKey()
-      .expect("Can't fetch local private key"),
+    localPublicKey: pkRes.get(),
   )
   secio.init()
   secio