AWS IID/IAM plugin documentation

[kfox1111]

There are multiple plugins that talk to AWS. The documentation for the plugins kind of assumes the auth will just work. But we're getting reports that, at least in Kubernetes, normal pods are not able to get credentials automatically. So spire-server doesnt work. We need more details on how to configure this so we can add proper support to the helm charts.

[amartinezfayo]

Thank you @kfox1111 for opening this. I think that we can have better documentation on this. We should probably document how the plugins gets the credentials according with this: https://aws.github.io/aws-sdk-go-v2/docs/configuring-sdk/