fix(NODE-5944): make AWS session token optional

[alenakhineika]

Description

According to spec:

Users MAY have obtained temporary credentials through an AssumeRole request. If so, then in addition to a username and password, users MAY also provide an AWS_SESSION_TOKEN as a mechanism_property.

The AWS_SESSION_TOKEN is optional, so we should not check for it when making temp credentials: https://github.com/mongodb/node-mongodb-native/blob/main/src/cmap/auth/mongodb_aws.ts

What is changing?

Do not throw the MongoMissingCredentialsError('Could not obtain temporary MONGODB-AWS credentials') exception when the AWS token is missing.

Is there new documentation needed for these changes?

None

What is the motivation for this change?

NODE-5747

Release Highlight

The AWS token is now optional

Users may provide an AWS_SESSION_TOKEN as a client option or AWS configuration in addition to a username and password. But if the token is not provided, the driver won't throw an exception and let AWS SDK handle the request.

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