NbAuthSocialLink allow configuration of icon pack

[lukas-mertens]

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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