Reproducible code as cohortBuilder code

[galachad]

Base on cohortBuilder metadata generete repro code using cohortBuilder code.

Example:

cohort(
  filter(
    "discrete", id = "author", dataset = "books", 
    variable = "author", value = "Dan Brown"
  ),
  filter(
    "range", id = "copies", dataset = "books", 
    variable = "copies", range = c(5, 10)
  ),
  filter(
    "date_range", id = "registered", dataset = "borrowers", 
    variable = "registered", range = c(as.Date("2010-01-01"), Inf)
  ) 
)

This code should be returned by get_state or code function.

I think, the best way of having that will be to support multiple type of output for repro code.

@krystian8207 could you share your though about that. What will be the best way to implement. Using existing code functionality or create a new function?