feat(context): introduce context events and observers for bind/unbind

[raymondfeng]

Spin-off from #2122

• Add support for context to emit bind and unbind events
• Allow observers to register with the context chain and respond to come and go of bindings of interest

Followed-up PRs:

#2122 (depends on #2291)

• Introduce ContextView to dynamically resolve values of matching bindings
• Introduce @inject.view to support dependency injection of multiple bound values
• Extend @Inject and @inject.getter to support multiple bound values

#2249

• Add an example package to illustrate how to implement extension point/extension pattern using LoopBack 4’s IoC and DI container

#2259

• Propose new APIs for Context to configure bindings
• Add @inject.config to simplify injection of configuration

Checklist

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated