chore: improve Listener signature

[hacksparrow]

Improved Listener signature so it can accept callback or promise.

Addresses https://github.com/strongloop/loopback-next/pull/4730/files#r385912758, required for landing PR loopbackio/loopback-next#4730.

The observe and removeObserver methods of a class implementing ObserverMixin can accept a callback or a promise. The current typing does not allow that. This PR re-defines the Listener type to be either a callback that returns void or a promise that resolves to void.

The problem this PR solves manifests only in TS projects, like LB next, hence it cannot be tested in the Juggler repository.

Before settling on this change, we tried another approach:

export interface Listener<Ctx = OperationHookContext<typeof ModelBase>> {
  (ctx: Ctx, next: (err?: any) => void): void;
  (ctx: Ctx): Promise<void>;
}

but the overloaded interfaced did not work as expected.

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• Commit messages are following our guidelines

[agnes512]

@slnode test please

[agnes512]

Hope jenkins passes

[hacksparrow]

@agnes512 @dhmlau Jenkins failing due to an un-related issue.

20:37:21 
20:37:21   ․․․․Connection fails: Error: connect ECONNREFUSED 127.0.0.1:4
20:37:21 It will be retried for the next request.
20:37:21 ․․․[ioredis] Unhandled error event: Error: connect ECONNREFUSED 127.0.0.1:6
20:37:22     at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1191:14)
20:37:22 ․,,․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․,,․․․․․․․․․!
20:37:26 
20:37:26   48 passing (6s)
20:37:26   4 pending
20:37:26 
  1 failing
20:37:26 
20:37:26   1) "before each" hook: clearDatabase for "works for Date values":
20:37:26      Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.