Update IAMUserPolicyAttachment to refer IAMPolicy

[sahil-lakhwani]

What problem are you facing?

This IAMUserPolicyAttachment resource currently can refer to a IAM Policy in AWS by it's ARN. This issue is to update IAMUserPolicyAttachment so that it can refer to a IAM Policy in AWS voia an IAMPolicy resource in Crossplane.

How could Crossplane help solve your problem?

There is already a resource in Crossplane called IAMPolicy. So, IAMUserPolicyAttachment can refer the IAMPolicy and use it's ARN (found as the value of crossplane.io/external-name aanotation).

[marlenepereira]

This feature and/or issue #177 would be great to have and really useful since we usually need to create an IAM policy suitable for each case following least privilege principle. Hopefully we can have these soon.

[sahil-lakhwani]

This has been addressed in #257 and can be closed.