NbAuthSocialLink allow configuration of icon pack

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**

It is not possible to configure the icon pack used for NbAuthSocialLinks

**Expected behavior:**

It should be possible to configure the icon pack used.

**Related code:**

For example I want to add a google and apple sign-in:
```
const socialLinks: NbAuthSocialLink[] = [
{
link: Routes.socialLogin.google,
icon: 'google',
title: 'Google',
},
{
link: Routes.socialLogin.apple,
icon: 'apple',
title: 'Apple',
},
];
```
As eva-icons doesn't contain the apple-icon, it is not trivial to add it from another icon-pack (I haven't found out a clean way yet).

If I would add support for this, e.g. add an optional attribute "pack" to the NbAuthSocialLink type, which is correctly interpreted, would a PR like that be accepted?

**Angular, Nebular**
Angular 13.1.3, Nebular 9.0.0