Convert hook_captcha to Plugins

Problem/Motivation

Currently there is a hook_captcha() in the module:

Description of the hook

The hook_captcha() hook is the only required function if you want to integrate
with the base CAPTCHA module.
Functionality depends on the first argument $op:

• 'list': you should return an array of possible challenge types
  that your module implements.
• 'generate': generate a challenge.
  You should return an array that offers form elements and the solution
  of your challenge, defined by the second argument $captcha_type.
  The returned array $captcha should have the following items:
  $captcha['solution']: this is the solution of your challenge
  $captcha['form']: an array of the form elements you want to add to the form.
  There should be a key 'captcha_response' in this array, which points to
  the form element where the user enters his answer.
  An optional additional argument $captcha_sid with the captcha session ID is
  available for more advanced challenges (e.g. the image CAPTCHA uses this
  argument, see image_captcha_captcha()).

This way is not very comfortable for modules to provide new types of CAPTCHA and does not allow to decouple needed code for custom type of CAPTCHA from entire module.

Proposed resolution

For better architecture, and to follow a way in which Drupal provides ability to extend default functionality I suggest to convert hook_captcha to plugins architecture, where each module will provide own plugin with custom realization of captcha.

This make sense to do for version 8.2.x

User interface changes

No changes.

API changes

Yes, some API parts have to be changed.

Data model changes

No changes.