Conform RPCError to RPCErrorConvertible (#7)

* Conform RPCError to RPCErrorConvertible
* Add metadata and cause

Author: 0xTim

Sources/GRPCCore/RPCError.swift

@@ -342,3 +342,22 @@ extension RPCError {
     self.cause = convertible.rpcErrorCause
   }
 }
+
+@available(gRPCSwift 2.1, *)
+extension RPCError: RPCErrorConvertible {
+  public var rpcErrorCode: Code {
+    self.code
+  }
+
+  public var rpcErrorMessage: String {
+    self.message
+  }
+
+  public var rpcErrorMetadata: Metadata {
+    self.metadata
+  }
+
+  public var rpcErrorCause: (any Error)? {
+    self.cause
+  }
+}