Add schema_version to .opensearch-notifications-config index mappings and automatically upgrade logic

[zhichao-aws]

Is your feature request related to a problem?
Currently we only have one version of mappings for .opensearch-notifications-config index. However, as we plan to support more notification channels, we will have new mappings. Current code doesn't check the version of index mappings, neither update the mappings. This will cause problems for clusters upgraded from older notifications versions.

What solution would you like?
Like ism plugin, add schema_version to .opensearch-notifications-config index mappings and automatically upgrade logic. The upgrade logic is triggerred by CRUD request of .opensearch-notifications-config.

What alternatives have you considered?
N/A

Do you have any additional context?
N/A