Improve template helper functions: string/slice (#24266)

Improve template helper functions: string/slice (#24266)

Follow #23328

The improvements:

1. The `contains` functions are covered by tests
2. The inconsistent behavior of `containGeneric` is replaced by
`StringUtils.Contains` and `SliceUtils.Contains`
3. In the future we can move more help functions into XxxUtils to
simplify the `helper.go` and reduce unnecessary global functions.

FAQ:

1. Why it's called `StringUtils.Contains` but not `strings.Contains`
like Golang?

Because our `StringUtils` is not Golang's `strings` package. There will
be our own string functions.

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Co-authored-by: silverwind <me@silverwind.io>