replace ArrayList with HashSet for rpcMethodExists check (#5708)

Signed-off-by: thinkAfCod <q315xia@163.com>
Co-authored-by: Sally MacFarlane <macfarla.github@gmail.com>

ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/jsonrpc/RpcMethod.java

@@ -14,8 +14,8 @@
  */
 package org.hyperledger.besu.ethereum.api.jsonrpc;
 
-import java.util.ArrayList;
 import java.util.Collection;
+import java.util.HashSet;
 
 public enum RpcMethod {
   ADMIN_ADD_PEER("admin_addPeer"),
@@ -192,7 +192,7 @@ public enum RpcMethod {
   }
 
   static {
-    allMethodNames = new ArrayList<>();
+    allMethodNames = new HashSet<>();
     for (RpcMethod m : RpcMethod.values()) {
       allMethodNames.add(m.getMethodName());
     }