[MD] Allow user to mark a data source as default and persist that in the metadata

[BionIT]

Is your feature request related to a problem? Please describe.

For index pattern management, we can mark an index pattern as default. That default index pattern is one shown up when the discover page renders. We should do the same for data sources

Describe the solution you'd like
We need to let users to mark a data source as default in the create and edit page, there should be one default data source.
The default property for the data source need to be persisted in the saved object

Describe alternatives you've considered
Have no default data source or the top one following alphabetic order sorted and decided at the front end

Additional context

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

[BionIT]

@kgcreative Just created this issue, and please feel free to correct if anything is not correct

[zhyuanqi]

Sure. Will work on this

[BionIT]

Thanks @zhyuanqi for helping with this issue!

[bandinib-amzn]

Duplicate of #5868. Please close this issue as well when you close this.

[zhyuanqi]

Sure Let me close it