client: add GetCandidateRegisterPrice method

It's important for clients.

pkg/rpc/client/native.go

@@ -30,11 +30,20 @@ func (c *Client) GetNNSPrice(nnsHash util.Uint160) (int64, error) {
 
 // GetGasPerBlock invokes `getGasPerBlock` method on a native NEO contract.
 func (c *Client) GetGasPerBlock() (int64, error) {
+	return c.getFromNEO("getGasPerBlock")
+}
+
+// GetCandidateRegisterPrice invokes `getRegisterPrice` method on native NEO contract.
+func (c *Client) GetCandidateRegisterPrice() (int64, error) {
+	return c.getFromNEO("getRegisterPrice")
+}
+
+func (c *Client) getFromNEO(meth string) (int64, error) {
 	neoHash, err := c.GetNativeContractHash(nativenames.Neo)
 	if err != nil {
 		return 0, fmt.Errorf("failed to get native NEO hash: %w", err)
 	}
-	return c.invokeNativeGetMethod(neoHash, "getGasPerBlock")
+	return c.invokeNativeGetMethod(neoHash, meth)
 }
 
 // GetDesignatedByRole invokes `getDesignatedByRole` method on a native RoleManagement contract.

pkg/rpc/client/rpc_test.go

@@ -478,6 +478,18 @@ var rpcClientTestCases = map[string][]rpcClientTestCase{
 			},
 		},
 	},
+	"getCandidateRegisterPrice": {
+		{
+			name: "positive",
+			invoke: func(c *Client) (interface{}, error) {
+				return c.GetCandidateRegisterPrice()
+			},
+			serverResponse: `{"id":1,"jsonrpc":"2.0","result":{"state":"HALT","gasconsumed":"2007390","script":"EMAMDWdldEZlZVBlckJ5dGUMFJphpG7sl7iTBtfOgfFbRiCR0AkyQWJ9W1I=","stack":[{"type":"Integer","value":"100000000000"}],"tx":null}}`,
+			result: func(c *Client) interface{} {
+				return int64(100000000000)
+			},
+		},
+	},
 	"getDesignatedByRole": {
 		{
 			name: "positive",