feat(gettoken): Add callback capability in a backward compatibility fashion

[skyless]

Description

Making getToken asynchronous by adding an optional callback function, in a backward compatibility fashion.

References

#248

Testing

All 39 tests passing.

• This change adds test coverage for changed functionality

Worth mentioning, I had to change the tests a little bit: all tests were sharing the same req and res objects while some tests are asynchronous. It means these objects use by one test would have been changed by the next test before it could verify it. Now every test has their own objects.

• Fix indentation
• Fix duplicated attribute
• Fix unused variable

Checklist

• I have added documentation for changed functionality in this PR
• All active GitHub checks for tests, formatting, and security are passing

[jfromaniello]

Thank you, in v7.1.0 you can use an async getToken.