Feature: NbCard elevation (elevation)

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
Currently it is not possible to set NbCard shadow via a parameter and NbCards within cards will always have the same shadow, resulting in a weird visual experience because they will display the same "elevation".

**Expected behavior:**
By setting the elevation:number parameter on nb-card element, users will be able to manage the NbCard shadow and create consistent elevated structures.

**Steps to reproduce:**
Nest a NbCard within a NbCard to check or components using NbCards.

**Related code:**
A similar parameter like the current "size" could help set the NbCard elevation, simply rendering the related bootstrap 4 shadow class: https://getbootstrap.com/docs/4.3/utilities/shadows/

I am happy to go ahead with the required PR if this gets the go ahead.