How can extensions introspect application artifacts

[bajtos]

This story is extracted from #4099, where we researched existing LoopBack 3 components and various techniques they use.

Now we need to write documentation for extension authors to show how to solve the following use cases in LoopBack 4:

1. Get a list of all models (entity classes) used by the application

2. Get definition of a given model (information about properties, relations, mixins, etc.)

3. Get a list of all remote methods (REST API endpoints) provided by a given model.

4. Obtain a list of all REST endpoints provided by the application, including metadata about request parameters & response schema, and any other information necessary to configure reverse-proxy routing rules (Kong, nginx,
   etc.).

In most cases, the new content will be useful to authors building new LB4 components too, therefore we should structure the content in two parts:

• A guide explaining how to build a component contributing a service, this should go to Extending LoopBack
• A guide explaining how to migrate LB3 components, this guide should go to Migrating components and extensions and focus on aspects unique to extensions migrating from LB3 and refer to content in Extending LoopBack wherever possible & appropriate.

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