Make hash function in signature settable

[nanjj]

In case apollo server using sha256 hash function as signature, the signature function can support also.

[mergify[bot]]

感谢您提出Pull Request，我会尽快Review。我会在1-2日内进行查看或者回复，如果遇到节假日可能会处理较慢，敬请谅解。

[nobodyiam]

This is an interesting proposal, however, t's hard to envision a scenario where apollo server would change the signature function any time soon...

[nanjj]

This is an interesting proposal, however, t's hard to envision a scenario where apollo server would change the signature function any time soon...

Suppose user changed https://github.com/apolloconfig/apollo-java/blob/07ed79b06859202a8e7df6fd007168182417df0c/apollo-core/src/main/java/com/ctrip/framework/apollo/core/signature/Signature.java#L40, and packaged apollo server, the apollo java client would work OK, while the agollo would not.