SMN component-support custom service endpoint

In current SMN component, we get endpoint of  the service using the 

SmnRegion.valueOf(region).getEndpoint() method in huaweicloud-sdk-smn-3.0.32, while the SmnRegion.valueOf() method restricts the region to Huawei Public Cloud regions. In some situation, sunch as  Huwei Private Cloud, the regions are differernt from those in Public Cloud, and we cannot use the method anymore, so we need to customize the endpoint address. 

According to the '3. Client Initialization' part in document of SMN SDK, it supports two ways to get endpoint, through region and through  specified endpoint. We can add a new parameter In SMN component , which can enable users to use a custom SMN endpoint.