添加 SumDB 代理支持，更新文档和测试用例

[lework]

SumDB Proxy

By default, sumdb (Checksum Database) requests use the same proxy host specified by the -proxy flag. You can specify a different sumdb proxy using the -sumdbProxy flag:

./bin/goproxy -listen=0.0.0.0:80 -proxy https://goproxy.io -sumdbProxy https://goproxy.cn

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