(Query assist) change isEnabled to an observable based on selected connection

[joshuali925]

previously query assist reads the datasource id from the selected index pattern. this PR updates it to read from the new ConnectionsService

• fix!: remove dataSource key in get connection response
  • this is a breaking change on the return response of GET ${API.DATA_SOURCE.CONNECTIONS}/{dataSourceId} to be consistent with type
• change isEnabled to an observable based on selected connection
  • this depends on https://github.com/opensearch-project/OpenSearch-Dashboards/pull/7183, should go in with that PR

[kavilla]

merging for now to unblock since the main repo got merged