[MDS] Refactor several utility functions in data source plugin

[huyaboo]

Is your feature request related to a problem? Please describe. Currently, there are two functions being repeated in plugins that can simply be extracted into data source plugin utility functions:

• A function that decides whether to use local cluster or remote OpenSearch client based on data source id
• A function that retrieves the data source id based on data source title/name

Describe the solution you'd like There should be three utility functions placed in the data source plugin that will be exposed so that other plugins can consume:

• decideClient(context: RequestHandlerContext, dataSourceId?: string, withLongNumeralsSupport: boolean): return the correct client
• decideLegacyClient(context: RequestHandlerContext, dataSourceId?: string): return the correct legacy client
• fetchDataSourceNameById(client: SavedObjectsClientCommon, dataSourceTitle: string): return the data source id (if it exists)
  • SavedObjectClientWrapper should be defined in both public and server folders for passing type checks

Describe alternatives you've considered

N/A

Additional context

N/A