feat(module:cascader): support for load options with observable

[HyperLife1119]

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 #8048 (50f4ccc) into master (fa0312a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 50f4ccc differs from pull request most recent head b2dd8c8. Consider uploading reports for the commit b2dd8c8 to get more accurate results

@@           Coverage Diff           @@
##           master    #8048   +/-   ##
=======================================
  Coverage   91.64%   91.65%           
=======================================
  Files         515      515           
  Lines       17640    17637    -3     
  Branches     2790     2788    -2     
=======================================
- Hits        16167    16166    -1     
+ Misses       1175     1173    -2     
  Partials      298      298           

Files Changed	Coverage Δ
components/cascader/cascader.component.ts	97.94% <ø> (ø)
components/cascader/typings.ts	100.00% <ø> (ø)
components/cascader/cascader.service.ts	100.00% <100.00%> (+0.48%)	⬆️

... and 1 file with indirect coverage changes