Plugin should use AWS SDK Credential Resolution Order

Right now it is impossible to use the AWS plugin without specifying a key via the API. When running boundary controllers on AWS instances this is a documented anti-pattern and the instance profile should be used instead.

What would it take to get this changed? I can put together a patch if there's good documentation around this.

Start by locating the plugin's AWS credential setup and compare it with the AWS SDK Credential Resolution Order. The change is complete when the plugin can run on an AWS instance using its instance profile without requiring an API-supplied key; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.