Adds a format_display_using option for the number field

[zhephyn]

This PR adds a format_display_using option for the number field such that instead of this:

field :amount, as: number, format_using: -> do
  if view == :index || view == :show
    number_to_currency value
  else
    value
  end
end

We can do this instead which is more cleaner and allows us to apply appropriate formatting on a value for both the index and show views at once :

field :amount, as: number, format_display_using: -> { number_to_currency value }

Solves #1902

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[codeclimate]

Method value has a Cognitive Complexity of 6 (exceeds 5 allowed). Consider refactoring.

[codeclimate]

Code Climate has analyzed commit 1d4abd0 and detected 1 issue on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	1

View more on Code Climate.

[zhephyn]

@Paul-Bob . I have a working solution so far. Before i proceed with writing tests for this particular feature and solving the failed CI checks, i wanted check if this is going in the expected direction. If yes, I'll proceed to write the tests and then update the docs.

Also, i opted for implementing just the format_display_using option alone since it allows us to do formatting for both the index and show views at once, instead of implementing individual options named format_index_using and format_show_using for the index and show page respectively.
In case these had to be implemented as well, kindly let me know and I'll make the necessary changes.