feat(module:TreeSelect): optional debounce

[zomixi]

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
[x] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #3437

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[codecov]

Codecov Report

Merging #3444 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3444      +/-   ##
==========================================
+ Coverage    95.6%   95.61%   +<.01%     
==========================================
  Files         609      609              
  Lines       13250    13265      +15     
  Branches     1810     1813       +3     
==========================================
+ Hits        12668    12683      +15     
  Misses        197      197              
  Partials      385      385

Impacted Files	Coverage Δ
components/tree-select/nz-tree-select.component.ts	99.11% <100%> (+0.06%)	⬆️
components/time-picker/time-holder.ts	97.26% <0%> (ø)	⬆️

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