CLI command to expose REST API of a Model with no custom classes

[bajtos]

Part of From model definition to REST API with no custom repository/controller classes #2036

A new CLI command that will ask the user for an existing model, an existing datasource, and then generate the corresponding public-models entry (model configuration json)

Nice to have: the pattern (crud vs key-value) can be automatically inferred from the datasource config (connector type).

(Much can be borrowed from lb4 repository)

Acceptance Criteria

• Introduce a new command modelconfig in the CLI to generate the model configuration json that will be stored in the public-models directory ( modelconfig is a suggestion)
• Update Generating LoopBack Artifacts section in documentation : https://loopback.io/doc/en/lb4/Command-line-interface.html#generating-loopback-artifacts. Be sure to update the sidebar as well.
• Provide new unit, integration, acceptance mocha tests

[dhmlau]

@bajtos, could you please add the acceptance criteria? Thanks.

[emonddr]

@bajtos I added the Acceptance Criteria. Please adjust it to your liking; if necessary. thx

[nabdelgadir]

Closing because #4932 is landed.