[CTB] Add format examples in date field select

[soupette]

What does it do?

It adds format examples in date field select:

Why is it needed?

So users have information about the format they are choosing.

How to test it?

1. Go to the CTB
2. Click on add new field and choose the Date one
3. See the date format example

[codecov]

Codecov Report

Merging #12871 (489f7a3) into master (f880ec4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12871   +/-   ##
=======================================
  Coverage   47.78%   47.78%           
=======================================
  Files         231      231           
  Lines        8619     8619           
  Branches     1922     1922           
=======================================
  Hits         4119     4119           
  Misses       3701     3701           
  Partials      799      799           

Flag	Coverage Δ
front	?
unit	47.78% <ø> (ø)

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