eks-pod-identity-agent session name for use with MSK

[metropolis-ameer]

eks-pod-identity-agent needs the ability to set a static session name for the role in use rather than generating a random session name. This creates an issue where the eks-pod-identity-agent refreshes the role credentials and causes a fatal error while continuing to connect to MSK because MSK expects the same session name.
see: aws/containers-roadmap#2362