Allow profile override for shared config sessions

[tydavis]

Permits use of AWS_PROFILE environment variable to use the Session shared config. If environment variable is not present, follows the old code path.

[tydavis]

This also vendors the updated code required to build (including https://github.com/tydavis/gobundledhttp). Bundled certs mean that regardless of on-disk certs, the ones in the library will always be trusted.