Authentication with Cognito about dce-cli HOT 2 OPEN

@joshmarsh is going to drive the github.com/Optum/dce work
@eschwartz is going to drive the github.com/Optum/dce-cli work

from dce-cli.

/auth endpoint encodes the STS token as base64
/auth endpoint displays base64 token to user, and copies to clipboard

@eschwartz , here's an example of the sts creds JSON:

{
   "accessKeyId":"xxx",
   "secretAccessKey":"xxx",
   "sessionToken":"xxx",
   "expireTime":"Wed Nov 20 2019 13:30:13 GMT-0600 (Central Standard Time)"
}

from dce-cli.