Enhance developer experience in persistence story

[dhmlau]

Description

Comparing the developer experience for LB3 and LB4 regarding steps involved to get REST APIs up and running that connects to database, it looks like there is room for improvement.

Current Behavior

Below is the developer experience in the persistance story

	LoopBack 3	LoopBack 4	Issue opened
Scaffold LB app	Run lb command	Run lb4 command	--
Define datasource	Run lb datasource command. datasources.json is generated.	manually either create a DataSource class or DataSource+datasource.json	#1221
Define model	Run lb model command. Bind it with datasource	Create model.ts manually	#1225
Bind model with datasource	Done in lb model command	Define Repository to bind datasource and model manually
Extra steps	none	Change Application class to extend from BootMixin(RepositoryMixin(RestApplication))
Run the app	npm start	npm start	--

[dhmlau]

Closing as done