Document extending context using `App<CustomContext>` generic

Description

In the types files for context that gets passed around, it's mentioned that we can add stuff to the context as a way of passing around information. Express does something similar with their Request/Response/Application, but they specifically allow you to extend their interfaces to add your own typed properties. For typescript, this is very handy.

Here's there global namespace that makes it possible: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/express-serve-static-core/index.d.ts#L15

Here's how you can add custom typed properties in a consuming app

declare global {
  namespace Express {
    interface Request {
      context: Context;
      foo: Foo;
    }
  }
}

which is really nice because the autocomplete works and it knows all my typings

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

Expected result:

Ability to add types to properties that have been defined in the global namespace to things that are typically passed through handlers, Context would be my suggestion, but other interfaces would also be welcome.

Extending the interface like above should then smartly apply the correct types to extra properties that people define.

Actual result:

Types are not applied today. Any extra properties assigned to the context variable are typed any. To get typing back, we have to force casting to a class/interface.

Attachments:

Start with the TypeScript context type files mentioned in the issue and compare their extension needs with Express's Request/Response/Application declarations. Define how consumers should provide typed custom context properties, then verify that handler context properties receive those types and no longer fall back to any.