lb4 service: support connectors not included in LoopBack's official list

Suggestion

A connector such as the sendgrid loopback connector, is a connector that does not require CRUD commands and it is not a REST data source. This currently seems to make it impossible to run the lb4 service command against such a connector, as the generator will expect a REST datasource.

My suggestion would be to add a new datasource metadata for a connector such as this (one that doesn't require CRUD and is not a REST api), so the service generator is able to recognize it as a valid datasource and make its methods available to controllers.

Use Cases

This will be useful for connecting a service to an email library such as sendgrid, or to utilize other libraries that provide a high-level API for their REST implementations. This includes just about any 3rd party library that is not controlled via REST directly.

Examples

1. Create a sendgrid email datasource based on one of the existing sendgrid loopback connectors
2. Run the lb4 service command, to tie this datasource to a service
3. Inject the service into a controller, to give the user the ability to send emails based on some API request

Acceptance criteria

based on https://github.com/strongloop/loopback-next/issues/3288#issuecomment-507628571

• Modify the logic in lb4 service and lb4 repository and when we don't have metadata about the base model used by the connector, then assume the connector is compatible with both services and repositories. Maybe print a hint that we don't know if the connector is compatible.

Hints:

• lb4 repository is already treating unknown connectors as compatible: https://github.com/strongloop/loopback-next/blob/b0a84b5adc30cac3614ea90e414b59cf76009ecf/packages/cli/generators/repository/index.js#L265-L276
• lb4 service interprets "unknown" as "incompatible": https://github.com/strongloop/loopback-next/blob/b0a84b5adc30cac3614ea90e414b59cf76009ecf/packages/cli/generators/service/index.js#L191-L196
• Changing lb4 service to return result !== false should fix the issue.
• The missing piece is how to write a test to verify the fix. The test should go to file remote-service.integration.js, take a look at the test for lb4 repository for inspiration here

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Read packages/cli/generators/service/index.js and the corresponding unknown-connector handling in packages/cli/generators/repository/index.js. Then inspect packages/cli/test/integration/generators/remote-service.integration.js and the repository integration test for its test structure. Done means the service generator accepts an unknown connector and the integration test verifies that behavior.