fixes #2689 Make the external url of cache server configurable if necessary (#2690)

* Make the external url of cache server configurable if necessary

Signed-off-by: Zoupers <qy@zouper.cn>

* optimize code and doc and fix test and add test

Signed-off-by: Zoupers <qy@zouper.cn>

* optimize code and adjust description

Signed-off-by: Zoupers <qy@zouper.cn>

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Signed-off-by: Zoupers <qy@zouper.cn>
Co-authored-by: 林玮 (Jade Lin) <linw1995@icloud.com>