Make the IAM Principal's ARN available to the shell environment

[helloPiers]

Opening this PR as its easier to explain what I mean this way, interested if this is something the EIC team would like to integrate or not.

Description of changes:
Usings SSH's feature where authorized_keys format can contain controlling data before the key, an environment variable is set up which will make the $caller (IAM Principal's ARN) available within the login session.

One must enable this feature in the sshd config, I have used a prefixed wildcard, but it could be a single value.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[helloPiers]

Interested in your thoughts please @LordAlfredo, thanks.

[VikashKumarInd]

Thanks for bringing this up. It seems to be a good feature support for the EIC customers. Considering that, we have added this feature request in our backlog where we will evaluate this option and accordingly make decision on whether we want to support this. Will keep you posted when we arrive at any decision.