fix: add missing return value to `fromAPIKey`

[hansemannn]

According to the docs, the fromAPIKey function should return an instance of the class again, but returns void in the actual code.

To properly use it in chaining scenarios, e.g. const client = new JWT().fromAPIKey('<api-key>'), this issue should be addressed.

[danielbankhead]

The docs are referencing GoogleAuth, which generates a JWT:

https://github.com/googleapis/google-auth-library-nodejs/blob/0b78d91534d97b37859a2303b332f8ccd52dbf69/src/auth/googleauth.ts#L806-L817

Sample usage:

import {GoogleAuth} from 'google-auth-library';

const key = '';

const auth = new GoogleAuth();
const client = auth.fromAPIKey(key);