feat(input-number): support input-number placeholder

[vthinkxie]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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
[ ] 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

close #1496

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1512 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1512      +/-   ##
==========================================
+ Coverage   96.92%   96.92%   +<.01%     
==========================================
  Files         222      222              
  Lines        9506     9507       +1     
  Branches     1297     1297              
==========================================
+ Hits         9214     9215       +1     
  Misses         42       42              
  Partials      250      250

Impacted Files	Coverage Δ
...mponents/input-number/nz-input-number.component.ts	98.99% <100%> (ø)	⬆️

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