client: add gasperblock getter

It's an important chain parameter.

pkg/rpc/client/native.go

@@ -25,3 +25,12 @@ func (c *Client) GetNNSPrice() (int64, error) {
 	}
 	return c.invokeNativeGetMethod(nnsHash, "getPrice")
 }
+
+// GetGasPerBlock invokes `getGasPerBlock` method on a native NEO contract.
+func (c *Client) GetGasPerBlock() (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")
+}

pkg/rpc/client/rpc_test.go

@@ -464,6 +464,18 @@ var rpcClientTestCases = map[string][]rpcClientTestCase{
 			},
 		},
 	},
+	"getGasPerBlock": {
+		{
+			name: "positive",
+			invoke: func(c *Client) (interface{}, error) {
+				return c.GetGasPerBlock()
+			},
+			serverResponse: `{"id":1,"jsonrpc":"2.0","result":{"state":"HALT","gasconsumed":"2007390","script":"EMAMDWdldEZlZVBlckJ5dGUMFJphpG7sl7iTBtfOgfFbRiCR0AkyQWJ9W1I=","stack":[{"type":"Integer","value":"500000000"}],"tx":null}}`,
+			result: func(c *Client) interface{} {
+				return int64(500000000)
+			},
+		},
+	},
 	"getMaxNotValidBeforeDelta": {
 		{
 			name: "positive",