Introduce getModelSchema and soft-deprecate getModelSchemaRef

Opening a follow up issue according to: https://github.com/strongloop/loopback-next/issues/4328#issuecomment-571638433

Cross posting @bajtos' comment from the above link:

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After we land feat(rest): add openapi schema consolidation #4365, I'd like to start the process of deprecating getModelSchemaRef and reworking both our existing code and CLI templates to leverage the new schema consolidation:

- Introduce getModelSchema similar to getModelSchemaRef (see src/controller-spec.ts)
- Rework our controller templates to use getModelSchema instead of getModelSchemaRef
- Rework controllers inside /examples to use getModelSchema
- Soft-deprecate getModelSchemaRef in our documentation (both /docs files and the tsdoc comment)
- Update https://github.com/strongloop/loopback4-example-shopping to use getModelSchema
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cc @bajtos @jannyHou

Start with src/controller-spec.ts and the linked schema-consolidation context to understand the requested getModelSchema change. Then inspect the controller templates, /examples controllers, /docs files, and the tsdoc comment; done means those usages and documentation use getModelSchema, with the external shopping example also updated.

Written by the indexing model from the issue text.