feat(bedrock): support AWS_SHARED_CREDENTIALS_FILE for autoload

[IdoKendo]

What does this PR do?

This PR enables AWS Bedrock provider autoload when AWS_SHARED_CREDENTIALS_FILE environment variable is set.

Previously only AWS_PROFILE, AWS_ACCESS_KEY_ID, and AWS_BEARER_TOKEN_BEDROCK triggered autoload.

The AWS_SHARED_CREDENTIALS_FILE is documented here: https://docs.aws.amazon.com/cli/v1/userguide/cli-configure-envvars.html

How did you verify your code works?

Aside from testing the change manually on my local env, this PR also adds test coverage for the new environment variable configuration.