Routing: Add regexp syntax support to UserMatcher

[vnxme]

This PR implements regexp syntax support to UserMatcher, i.e. allows to match user email in routing rules with regular expressions using regexp:{pattern} syntax. It has also been requested in #2536.

[APT-ZERO]

Thank you, it's useful , You can also add support for strings.Contains(), strings.HasPrefix(), strings.HasSuffix() as they are faster and uses less resources
for 'user' and also for 'inboundTag'

[Fangliding]

最好找时间以一个更统一的方法把这些东西全实现regexp 不然后面如果又有人加然后方法又不一样乱七八糟发

[Fangliding]

好吧翻了一下也就inbound tag和user可以加 user又是唯一值得加的

[yuhan6665]

感谢 @vnxme ！
@APT-ZERO ，regex 已经满足需求 所以不需要再加其他
@Fangliding 是的如果有超过三个地方需要 以后可以考虑 refactor