Allow community extensions to contribute example projects

[bajtos]

As a follow-up for #836, we should enhance lb4 example to be able to download examples from different GitHub repos, so that community-maintained extensions like gRPC, MQTT and GraphQL can contribute examples too.

Acceptance criteria

• Work with the community maintainers to create at least one example project in each extension:
  • https://github.com/strongloop/loopback4-extension-grpc
  • https://github.com/strongloop/loopback4-extension-typeorm
  • https://github.com/strongloop/loopback4-extension-mqtt
  • https://github.com/strongloop/loopback4-extension-graphql
• Enhance our CLI to be able to fetch examples from different GitHub repositories. Configure the CLI with the example created in the previous step.

[stale]

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