Fix return type in AMQPConsumer constructor

[JoshJarabek7]

Originally only had void as the return type, and the example on the README.md showed a Promise being returned, causing yellow squiggles from Typescript / Sonarlint.

[baelter]

Isn't that because your onMessage callback returns a Promise. In that case I guess we should change to any in the lib since we can't control what users of the lib return in their callbacks. Or the lib should handle async callbacks.. WDYT @carlhoerberg?