feat(module:dropdown): support customize icons

[hsuanxyz]

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

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[codecov]

Codecov Report

Merging #3517 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3517      +/-   ##
==========================================
+ Coverage    95.6%   95.61%   +<.01%     
==========================================
  Files         609      609              
  Lines       13250    13251       +1     
  Branches     1810     1810              
==========================================
+ Hits        12668    12670       +2     
  Misses        197      197              
+ Partials      385      384       -1

Impacted Files	Coverage Δ
components/dropdown/nz-dropdown.module.ts	100% <100%> (ø)	⬆️
...omponents/dropdown/nz-dropdown-button.component.ts	100% <100%> (ø)	⬆️
components/core/util/dom.ts	85.29% <0%> (+2.94%)	⬆️

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