Adding user-agent to IMDS requests

[joeysk2012]

In order to track metrics in IMDS we have to make identifying calls from this package with a unique User-Agent header.

Issue #, if available:

Description of changes:

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Tested the change using TCP dump:

        GET /latest/meta-data/network/interfaces/macs/02:ca:ba:d2:fd:37/local-ipv4s HTTP/1.1
        Host: 169.254.169.254
        User-Agent: amazon-ec2-net-utils/2.6.1
        Accept: */*
        X-aws-ec2-metadata-token:............

[joeysk2012]

Reviewed and approved internally.