feat: Firebase auth

[mishkolesnikov]

Please read and mark the following checklist 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:

Adds a new package nebular/firebase-auth with the dependencies of firebase and angular/fire.
The package provides strategies that allow nebular/auth to work with firebase auth.
Includes password, google, facebook and twitter strategies.

[codecov]

Codecov Report

Merging #2385 into master will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2385      +/-   ##
==========================================
+ Coverage   81.52%   81.54%   +0.02%     
==========================================
  Files         244      244              
  Lines        7377     7382       +5     
  Branches      773      774       +1     
==========================================
+ Hits         6014     6020       +6     
+ Misses       1135     1132       -3     
- Partials      228      230       +2     

Impacted Files	Coverage Δ
.../components/autocomplete/autocomplete.directive.ts	81.29% <66.66%> (-0.71%)	⬇️
...ramework/theme/components/toastr/toastr.service.ts	97.39% <0.00%> (+2.60%)	⬆️