Add workflow for OCI AWS tests & document the setup

[darkowlzz]

• Add a new workflow integration-aws for running the OCI AWS integration tests.
• Add setup docs for configuring the AWS account and GitHub repository for running the tests.

Depends on https://github.com/fluxcd/test-infra/pull/41.

:warning: The AWS and GitHub requirements as documented in the test README under IAM and CI setup needs to be fulfilled before or soon after merging this. Else, the CI jobs triggered due to the workflow cron configuration would fail.

Example run: https://github.com/darkowlzz/pkg/actions/runs/9276292128/job/25523036576

Part of https://github.com/fluxcd/flux2/issues/4619