aws - session policy support via cli

[PratMis]

Closes #9404
Took a stab at it and this is an initial draft. I can go ahead and add tests etc. However, have a few open questions from a design perspective

1. The CLI in this setup will only allow a json file. Haven't added a step where the json is validated because AWS by default gives an error at the time of assume_role if the session policy json is a MalformedPolicyDocument
   Tested with a bad json policy and the traceback received was
   botocore.errorfactory.MalformedPolicyDocumentException: An error occurred (MalformedPolicyDocument) when calling the AssumeRole operation: Syntax errors in policy.
2. Is it worth allowing inline policies? Imo, it can get pretty messy
3. We could potentially add a step for input validation re: whether the input is a json file or not?

I also tested it with a good session policy.

[update] - This PR supports passing in a session policy json document alongside assume role. When used, it will only allow a subset of permissions on the role assumed. The goal is to allow stakeholder use specific actions depending on what they feel comfortable with vs opening up the full role

[PratMis]

@kapilt , whenever you get a chance can i get one more review please?

[kapilt]

lgtm