support AWS Web Identity Token

[ruiyang2015]

our AWS setup is using the AWS Web Identity Token using AWS_WEB_IDENTITY_TOKEN_FILE, AWS_ROLE_ARN, and AWS_ROLE_SESSION_NAME environment variables.
And The AWS SDK for C++ does support WebIdentityTokenCredentialsProvider,
adding this will make https much easier to use.

[Tmonster]

related #16

[mst]

I've created a PR for this and successfully tested on an EKS pod with IRSA

@Tmonster can you please have a look? #136