Add support for POST /service/token in readonly mode

[stonezdj]

Usually, docker client sent GET request to /service/token. it seems that some other tools such as finch use POST
When Harbor is in readonly mode, the middleware will ban the POST/PUT method except readonlySkippers but it seems that the /service/token is not in the list, actually the POST method to /service/token is supported.

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[github-actions]

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[stonezdj]

This issue could be reproduced by deploy the image pulling from a read-only instance with secret, the output is

  Normal   Scheduled  23s   default-scheduler  Successfully assigned default/my-dep-74d57f54b5-lgzkr to kind-control-plane
  Normal   Pulling    22s   kubelet            Pulling image "xxxx/library/nginx:latest"
  Warning  Failed     13s   kubelet            Failed to pull image "xxx/library/nginx:latest": rpc error: code = Unknown desc = failed to pull and unpack image "xxxx/library/nginx:latest": failed to resolve reference "xxxx/library/nginx:latest": failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://xxxx/service/token: 403 Forbidden
  Warning  Failed     13s   kubelet            Error: ErrImagePull
  Normal   BackOff    12s   kubelet            Back-off pulling image "xxxx/library/nginx:latest"
  Warning  Failed     12s   kubelet            Error: ImagePullBackOff