feat(module:float-button): add float-button component

[OriginRing]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[zorro-bot[bot]]

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.18421% with 21 lines in your changes missing coverage. Please review.

Project coverage is 91.50%. Comparing base (b59928f) to head (c61cc43).

Files	Patch %	Lines
...nents/float-button/float-button-group.component.ts	80.00%	9 Missing and 2 partials :warning:
...ponents/float-button/float-button-top.component.ts	91.17%	5 Missing and 1 partial :warning:
components/float-button/float-button.component.ts	83.33%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7884      +/-   ##
==========================================
- Coverage   91.53%   91.50%   -0.03%     
==========================================
  Files         536      541       +5     
  Lines       18445    18597     +152     
  Branches     2907     2920      +13     
==========================================
+ Hits        16883    17018     +135     
- Misses       1242     1251       +9     
- Partials      320      328       +8     

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[NRaf]

Hi,

Just came across this feature in the React branch and noticed it's missing from ng-zorro. Any idea on when this will be merged in?