feat: add support for EC2 Instance Identity roles

[gilbahat]

What this PR does / why we need it:

This PR enables aws-iam-authenticator to support EC2 Instance Identity roles - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-identity-roles.html , by parsing their UserID format. This enables use cases such as limited dynamic ad-hoc access to a kubernetes cluster based on AWS identity.

as of now, this prints an error: "sts getCallerIdentity failed: malformed UserID "5553582100:aws:ec2-instance:i-1234567890"" method=POST path=/authenticate , this error is incorrect as the UserID is well formed (just not what was expected)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
fixes #696

[linux-foundation-easycla]

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• ✅ login: gilbahat / name: Gil Bahat (c76c460)

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[micahhausler]

Thanks for this PR! Can you open a GH issue so we can discuss the use case? I want to better understand why this is needed as an alternative to the EC2 instance role

[orsenthil]

This enables use cases such as limited dynamic ad-hoc access to a kubernetes cluster based on AWS identity.

Under what scenarios would be this useful ?