* feat: add the extension sub-commad to download it
* do not put store-ext in the main image
* support to fetch the latest image tag
* finish the basic feature of extension downloading
* add some unit tests
* support different image registry
* add more unit tests
* remove useless files
* fix the unit test case
* simplify the apt install in the dockerfile
* add explanation for the empty methods
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Co-authored-by: rick <LinuxSuRen@users.noreply.github.com>