Add AWS Account ID to capability namespace annotations

[rifisdfds]

We currently only have a KIAM role ARN added to capability namespace annotation when it is created which contains the AWS account number embedded in an ARN string. As KIAM is being deprecated and this will presumably be disappearing, we need just the AWS Account ID so that it can be used by the operator in #520 to annotate the ProviderConfig correctly, and can be used for other things in future.

• [x] Add dfds-aws-account-id Annotation on namespace creation via k8s-janitor
• [x] Test in sandbox
• [x] Create PR
• [x] Merge PR
• [x] Retrospectively add/convert dfds-aws-account-id to existing capability namespaces (perhaps a one off script to extract from the kiam-role-arn
• [x] Test script in sandbox
• [x] Run script against production

[rifisdfds]

PR for k8s-janitor: https://github.com/dfds/k8s-janitor/pull/56