feat(auth): add support for refresh tokens

[dizco]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Added a way to refresh tokens inside the AuthService. This is not yet hooked anywhere, but it could be hooked inside an interceptor, as I've seen mentionned in #260. Typically, I think the refresh token would be stored in a HttpOnly cookie. This also addresses #362.

this.authService.refreshToken('email').subscribe((result: NbAuthResult) => {
  //Do something
});

I have also fixed #389, which involved changing private getProvider to protected getProvider.

Please let me know if I need to change anything!

[nnixaa]

Hey @dizco, looks great to me! Very good addition. @Tibing what do you think?

[nnixaa]

Thanks again, @dizco. If by any chance you have other ideas we would be happy to discuss 😃

[dizco]

@nnixaa no problem! I'll let you guys know if I think of anything else 😃 I guess the best way moving forward would be to open issues right?

[nnixaa]

@dizco yes, exactly. Looking forward to it!