[Multiple Datasource] Create example plugin with multiple data source support

[BionIT]

Is your feature request related to a problem? Please describe.
Since 2.4.0, multiple data source feature is enabled as a core plugin(see RFC #1388 and PR #2334).
Data source plugin was released in OpenSearch Dashboards 2.4.0 as part of the multi data source feature.

In 2.13, we exposed a few components for dashboards plugins to consume in order for users to pick a data source and use it to render the page.

In the implementation of plugins to support multi data source, we found it hard to show how the integration can be done.

Describe the solution you'd like
An example plugin can provide better understanding in the context of how plugins can make the integration.

Describe alternatives you've considered
Just simply provide information or code example in developer guide

Additional context

Add any other context or screenshots about the feature request here.