feat(module: alert): support custom icon

[d2FuZ3h1ZG9uZw]

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: #7687
close: #7687

What is the new behavior?

Supported custom icon.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Perhaps a field describing which version the feature was added could be provided, like Ant Design React.

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7691 (c4222a2) into master (616f59f) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7691      +/-   ##
==========================================
+ Coverage   92.03%   92.06%   +0.03%     
==========================================
  Files         502      502              
  Lines       16766    16767       +1     
  Branches     2638     2638              
==========================================
+ Hits        15430    15436       +6     
+ Misses       1069     1062       -7     
- Partials      267      269       +2     

Impacted Files	Coverage Δ
components/alert/alert.component.ts	95.31% <100.00%> (+0.07%)	⬆️
components/table/src/table-style.service.ts	94.82% <0.00%> (-1.73%)	⬇️
components/table/src/table/tr-measure.component.ts	91.30% <0.00%> (+4.34%)	⬆️
...ts/table/src/table/table-inner-scroll.component.ts	83.82% <0.00%> (+4.41%)	⬆️
components/table/src/table/tbody.component.ts	100.00% <0.00%> (+11.11%)	⬆️

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[simplejason]

LGTM