Allow aws checks to use existing credentials without assumerole

[jesusfcr]

Motivation

The checks that need access to an AWS Account uses the vulcan assume role endpoint for request temporary credentials from an http server.
The implementation of that server is not open source so it limits the usage of those checks.
The code if scattered and duplicated for each check.

Description

This pr allows the checks to access AWS Accounts with standard methods:

1. If VULCAN_ASSUME_ROLE_ENDPOINT is set use the endpoint to retrieve the credentials.
2. Otherwise obtain the current credentials from the system and if ROLE_NAME exists assume that role.

This method allows to execute the checks without requiring the vulcan-assume-role service using the aws provided credentials

• Running in a pod with the required aws credentials
• Running a check in lava passing the aws credentials

TODO / DOUBT / WIP

The current solution unifies the code required to get aws.Config.

• The helper functions are in an internal package. Those could be moved to the aws-check-sdk. Although adding an internal pkg could be a good alternative for sharing code without requiring to upgrade the sdk.
• It reports INCONCLUSIVE if the check can not access.
• The helper functions could support a list of roles to assume.