feat(module:back-top): add nzDuration property

[cipchk]

• close [BackTop] support duration  #5887

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
[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
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #5892 into master will increase coverage by 0.06%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #5892      +/-   ##
==========================================
+ Coverage   90.02%   90.09%   +0.06%     
==========================================
  Files         463      463              
  Lines       13941    13956      +15     
  Branches     2204     2211       +7     
==========================================
+ Hits        12551    12573      +22     
+ Misses        855      846       -9     
- Partials      535      537       +2     

Impacted Files	Coverage Δ
components/core/services/scroll.ts	83.60% <66.66%> (+14.85%)	⬆️
components/anchor/anchor.component.ts	97.11% <100.00%> (+0.02%)	⬆️
components/back-top/back-top.component.ts	93.18% <100.00%> (+0.15%)	⬆️
components/core/util/text-measure.ts	91.30% <0.00%> (+1.73%)	⬆️

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