Add support for regional Session Manager settings

[mattheidelbaugh]

Describe the feature

It would be useful to add a session-manager filter to the account resource to expose the SSM-SessionManagerRunShell SSM Document which "holds regional settings for Session Manager."

Extra information or context

My only concern is that in Cloudtrail all SSM Documents, including the SSM-SessionManagerRunShell Document this filter would be concerned with, are redacted, indicating that they can contain sensitive data we would not want to expose. The structure of this Session Manager document can be found here: https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager-schema.html. The only fields I see as potentially concerning are the shellProfile fields which contain "environment variables, shell preferences, or commands you want to run when your session starts." Would love feedback as to whether or not this is something Custodian should support before I start development.