feat(module: input-item): add label content api

[sWhite01111]

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-mobile/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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[x] Yes
[] No

Other information

[codecov]

Codecov Report

Merging #527 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
- Coverage   88.61%   88.61%   -0.01%     
==========================================
  Files         268      268              
  Lines        7539     7546       +7     
  Branches     1066     1066              
==========================================
+ Hits         6681     6687       +6     
  Misses        423      423              
- Partials      435      436       +1

Impacted Files	Coverage Δ
components/input-item/input-item.component.ts	93.68% <100%> (-0.29%)	⬇️

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