Add scan command.

[fujiwara]

ecrm scan scans existing resources, and outputs image URIs in use.

Usage: ecrm scan [flags]

Scan ECS/Lambda resources. Output image URIs in use.

Flags:
  -o, --output="-"            File name of the output. The default is STDOUT ($ECRM_OUTPUT).

And add ecrm plan/delete --scanned-files flag. ecrm does not delete images in these files.

Usage: ecrm plan [flags]

Scan ECS/Lambda resources and find unused ECR images that can be deleted safely.

Flags:
      --[no-]scan                          Scan ECS/Lambda resources that in use ($ECRM_SCAN).
      --scanned-files=SCANNED-FILES,...    Files of the scan result. ecrm does not delete images in these

Combine these flags, ecrm enables to support multi regions / accounts.
For example,

• The account A provides ECR.
• The account B runs ECS tasks that using ECR images in account A.

In this case,

1. ecrm scan --output in-use-b.json runs on account B.
2. ecrm delete --scanned-files in-use-b.json runs on account A. ecrm keeps images used by account B not only used in account A.

Multiple --scanned-files flags are allowed. So account A can keep images in use by multi accounts safety.