feat(module:statistics): countdown support finish event

[wzhudev]

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 #3902 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3902      +/-   ##
==========================================
+ Coverage   95.24%   95.25%   +0.01%     
==========================================
  Files         726      726              
  Lines       14672    14674       +2     
  Branches     1910     1910              
==========================================
+ Hits        13974    13978       +4     
+ Misses        260      259       -1     
+ Partials      438      437       -1

Impacted Files	Coverage Δ
components/statistic/nz-countdown.component.ts	90% <100%> (+5.78%)	⬆️

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[vthinkxie]

LGTM