feat(module:tree-select): add nzSelectedTemplate option

[powerleks]

Add nzSelectedTemplate option to support custom template of selected values

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
• 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?

The content of selected values in nz-tree-select component can not be customized.

Issue Number: N/A

What is the new behavior?

The content of selected values can be customized like in nz-select component.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.91%. Comparing base (2e0c78d) to head (5c30a6a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8923   +/-   ##
=======================================
  Coverage   91.91%   91.91%           
=======================================
  Files         554      554           
  Lines       19682    19684    +2     
  Branches     2936     2937    +1     
=======================================
+ Hits        18091    18093    +2     
  Misses       1265     1265           
  Partials      326      326           

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