refactor shared config Loading

[skotambkar]

PR for Shared Config Loading

Changes:

• Separates shared config and shared credentials files.
• Updates shared config handling to be consistent with AWS SDKs and CLI
• Updates test

TODO:

[ x ] More TESTING
[ x ] Error handling.
[ x ] Logs things that are dropped.
[ x ] Documentation update
[ x ] Config SourceFile should be a list of all filenames that were used to get Config