[#398] Support retryer

Add two strategy for PutBucketSettings request retryer:
* exponential backoff (increasing up to `max_backoff` delays with jitter)
* constant backoff (always the same `max_backoff` delay between requests)

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/handler/handlers_test.go

@@ -132,6 +132,18 @@ func (c *configMock) ResolveNamespaceAlias(ns string) string {
 	return ns
 }
 
+func (c *configMock) RetryMaxAttempts() int {
+	return 1
+}
+
+func (c *configMock) RetryMaxBackoff() time.Duration {
+	return 0
+}
+
+func (c *configMock) RetryStrategy() RetryStrategy {
+	return RetryStrategyConstant
+}
+
 func prepareHandlerContext(t *testing.T) *handlerContext {
 	return prepareHandlerContextBase(t, layer.DefaultCachesConfigs(zap.NewExample()))
 }