Add support for IMDS Version 2.

[michaelfoley1]

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html

[klauspost]

@michaelfoley1 Please check these linter/vet checks:

Running lint check
Error: pkg/credentials/iam_aws.go:62: File is not `goimports`-ed (goimports)
	tokenRequestTTLHeader = "X-aws-ec2-metadata-token-ttl-seconds"
	tokenPath             = "/latest/api/token"
	tokenTTL              = "21600"
	tokenRequestHeader    = "X-aws-ec2-metadata-token"
Error: pkg/credentials/iam_aws.go:263:8: ineffectual assignment to `err` (ineffassign)
	data, err := ioutil.ReadAll(resp.Body)
	      ^
Error: pkg/credentials/iam_aws_test.go:139:3: S1023: redundant `return` statement (gosimple)
		return
		^
Error: make: *** [Makefile:15: lint] Error 1
Error: Process completed with exit code 2.

[michaelfoley1]

@klauspost Sorry about that.
I've fixed any lint/vet issues and force-pushed over my branch (as per the CONTRIBUTING.md file)

[michaelfoley1]

@klauspost the tests are failing with the message:

GitHub Actions has encountered an internal error when running your job.

Could you try giving it a kick/retry?