refactor cache implement for function regexp_substr

[lcwangchao]

Enhancement

see #49464

regexp_substr caches some intermediate regexp.Regexp inside the function when some args are constant. Some issues for the current implement:

1. The cache logic is coupled with plan cahe, and it is hard to say the function is stateless.
2. When plan cahe is enabled, we cannot cache the object regexp.Regexp some times. For example, when the regex pattern is from the variable.