feat: allow linking existing python interpreters to managed location

[frostming]

Signed-off-by: Frost Ming me@frostming.com

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.61%. Comparing base (dc8e3f4) to head (99daf1c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pdm/cli/commands/python.py	94.59%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3215      +/-   ##
==========================================
+ Coverage   85.60%   85.61%   +0.01%     
==========================================
  Files         112      112              
  Lines       11224    11251      +27     
  Branches     2435     2440       +5     
==========================================
+ Hits         9608     9633      +25     
  Misses       1101     1101              
- Partials      515      517       +2     

Flag	Coverage Δ
unittests	85.42% <94.59%> (+0.01%)	⬆️

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