NODE-3152: ensure AWS environment variables are applied properly

[rose-m]

Description

The AWS environment variables were not detected properly.

The checks for username == null and password == null would always return false as the static merge(...) method that is used to create an instance of MongoCredentials defaulted to an empty string as value for both.

Furthermore, the AWS_SESSION_TOKEN must be applied if it is present in the environment variables and takes precedence over any other value. See https://github.com/mongodb/specifications/blob/master/source/auth/auth.rst#environment-variables:

However, if AWS_SESSION_TOKEN is set Drivers MUST use its value as the session token.

The direct assignment of AWS_SESSION_TOKEN would also potentially fail as it happened that this.mechanismProperties was initialized with an already freezed object throwing an error. That's why I changed that to a destructured assignment.

With those changes I am able to run all my E2E tests in mongosh for AWS authentication successfully.

Please note that I did not know where to add test cases for the driver and would ask you to either give me a hint so I can add them or add them on your own :)

[durran]

LGTM

[nbbeeken]

LGTM Thanks @rose-m! I'll merge this soon (edit: Actually I've enabled the AWS Auth tests, so I'll wait on those, and we still have some spec convo above)

Just a heads up to the team: Looks like we need to back port this to 3.6 so I'll put that metadata into the ticket.