[Image v3] Add copyright support

[renow-luxembourg]

Feature Request

Is your feature request related to a problem? Please describe.
Photographs, illustrations and other images will generally be protected by copyright as artistic works.
We would like to add some copyright/credits information under each image (in addition of the caption) with a différent style.
For the moment in the DAM we can define a caption (titleValueFromDAM) and an alternative text (altValueFromDAM)

Describe the solution you'd like
The easier solution for us will be to add a new field in the DAM named copyright.
And display it like the caption :

<span class="cmp-image__title" itemprop="caption">Image caption</span>
<span class="cmp-image__credits" itemprop="caption">©Image copyright</span>

This will allow us to customize the style with css.

Are there alternatives?
An other alternative is to allow HTML code in the title field of the DAM. The result will be something like :

<span class="cmp-image__title" itemprop="caption">
  <span class="caption">Image caption</span>
  <span class="credits">©Image copyright</span>
</span>

But it require for the redactor to learn a bit of HTML