managed datasource instance migration

[yesoreyeram]

backend datasource.serve method migrated to datasource.manage method.

How to test

• build the frontend yarn build
• build the backend mage -v
• spin up the grafana docker compose up
• visit localhost:3000
• open the provisioned datasource Postman Echo - Basic Auth and perform the health check -This ensure check health code path works as expected
• open the provisioned datasource Infinity - Open the explore and run query.
• change to backend parser - should produce the results
• change the source to "reference data" and select "users.csv", change the type to "csv" - should produce the results. This ensure the resource calls and query path works as expected.

[gwdawson]

left one comment but otherwise LGTM!