user setting for AWS config/credentials file location

## Problem

I have a bunch of customer aws accounts I manage. Some customers will even have 10+ accounts with each having at least two separate profiles, ie different roles for the same account. Because of this I have a directory for each customer and use the `AWS_CONFIG_FILE` variable to point to a customer specific config file. The extension only allows me to use the default config file located at `~/.aws/config`.

## Expected behavior

I would like another option when selecting a connection to point to where the aws config file is. Versus only using the default which is expecting me to stuff my default config file with a few hundred profiles.

Maybe there is something in this extension I'm missing which allows this?

No source files or tests are named in the issue. Start by tracing how the AWS Toolkit loads the default ~/.aws/config and how connection selection resolves profiles; compare that path with the AWS_CONFIG_FILE variable described in the report. Done means a connection can select a customer-specific AWS config/credentials location instead of relying only on the default file.

Written by the indexing model from the issue text.