fix: create portal user for customer from shopping cart

[git-avc]

fix #36744

From shopping cart, customer is created when order/quotation is placed. But customer don't connect with user.
This PR adds user to portal_user child on customer form.

[codecov]

Codecov Report

Merging #36745 (41a5119) into develop (a34cb8a) will increase coverage by 0.23%.
Report is 21 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #36745      +/-   ##
===========================================
+ Coverage    64.94%   65.18%   +0.23%     
===========================================
  Files          792      792              
  Lines        61821    61836      +15     
===========================================
+ Hits         40151    40306     +155     
+ Misses       21670    21530     -140     

Files Changed	Coverage Δ
erpnext/e_commerce/shopping_cart/cart.py	52.39% <100.00%> (+0.12%)	⬆️

... and 13 files with indirect coverage changes