Support IAM instance profiles

[reedloden]

Instead of requiring an explicit access key and secret key, add support for IAM instance profiles via the EC2 metadata service to allow dynamically getting temporary credentials as needed.

http://aws.amazon.com/about-aws/whats-new/2012/06/11/Announcing-IAM-Roles-for-EC2-instances/
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html#instance-metadata-security-credentials

Examples of other PHP projects implementing this:

• https://secure.phabricator.com/D9782
• https://github.com/aws/aws-sdk-php/blob/33e8f7083fcb54fd926a8569cb81a0795e8c2633/src/Credentials/InstanceProfileProvider.php

[gwynnarth]

+1 I would love to see that implemented since requiring passing IAM secrets complicate our setup significantly.

[IvoJager]

If anyone finds it useful, this is slightly modified version of 0.92 that takes the temporary token and sends it along with the headers;

IvoJager@036f33b

It works for our purposes - no guarantees it will for your setups (though I can't see any reason it won't...).

[lgrassini]

+1. We should never have hardcode credentials in the codebase. Is there any status about implementing this into the library? @daniel-zahariev Thanks!