feat(NODE-6141): allow custom aws sdk config

[durran]

Description

Allows users to provide a custom AWS credential provider for use with MONGODB-AWS authentication.

What is changing?

• Allow passing a AWS_CREDENTIAL_PROVIDER auth mechanism property to fetch credentials.
• Passes user provided credential provider to the AWS auth provider.
• For auto-encryption, a awsCredentialProvider auto encryption option can also be provided with either the same credentials provider or even a different one.

Is there new documentation needed for these changes?

Updates to the AWS auth section in the driver manual.

What is the motivation for this change?

NODE-6141

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