feat: add aws assume role support for ssm

[syphernl]

This adds an optional aws_assume_role input which is used in the SSM tunnel.
It will assume a given role before starting the tunnel.

In our use-case we run Terraform in a separate account, which needs to assume a role into the target account before it can do any actions.
For normal Terraform this is handled using the assume_role functionality of the aws provider, but in this particular case we need to explicitly assume the role ourselves.

With these changes we were able to use the module without getting a "Permission Denied" error.