Honor `AWS_SHARED_CREDENTIALS_FILE` env variable

[yermulnik]

Is your feature request related to a problem? Please describe.
The aws-sso console command doesn't honor AWS_SHARED_CREDENTIALS_FILE env var.

Historically I use AWS_SHARED_CREDENTIALS_FILE to switch between different AWS CLI credentials files for different projects. It's got a mix of plain creds and SSO profiles and doesn't require AWS CLI profiles to be "split" between config and crdentials files. And it "just works", including SSO sessions too, as AWS CLI (and SDK) permits AWS CLI profiles in both config and credentials files (ref: https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-files.html#cli-configure-files-where + https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-files.html#cli-configure-files-format-profile)

Describe the solution you'd like
aws-sso console command honors and supports the AWS_SHARED_CREDENTIALS_FILE env var.

Maybe even make aws-sso globally support AWS_CONFIG_FILE and AWS_SHARED_CREDENTIALS_FILE env vars if doable and feasible.

Describe alternatives you've considered
At the moment I either need to revisit and convert several existing credentials files to config files and update/adjust tooling and scaffolding around them, or (and this is what I have to use instead of aws-sso console command) use alternative tools that honor the AWS_SHARED_CREDENTIALS_FILE env var to look up AWS CLI profile by name.

Additional context
This is similar to #540, but for another related env var.

Granted aws-sso-cli utilizes AWS SDK for Go, I hope it won't be a bit of a hassle to implement this 🤞🏻 Thank you.