[MD]Add support for opensearch legacy client for data source

[zhongnansu]

Need to do most things similar to opensearch data source client. Register interface, create route context, etc

By adding support for legacy client, I assume the route context for data source will look like this

datasource: {
  getClient：(id) => Promise<OpenSearchClient>
  legacy: {
    getClient: (id) => Promise<OpenSearchLegacyClient>
  }
}