Improve performance of URL.update_query

[bdraco]

What do these changes do?

• Avoid reparsing the query string into a MultiDict
• Avoid creation of many intermediate MultiDict and MultiDictProxy objects
• Build the final string with iterable helper since there will never be QueryVariable

Are there changes in behavior for the user?

no

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.15%. Comparing base (ee90b00) to head (9d1cb96).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1130      +/-   ##
==========================================
+ Coverage   95.12%   95.15%   +0.03%     
==========================================
  Files          30       30              
  Lines        4638     4647       +9     
  Branches      410      411       +1     
==========================================
+ Hits         4412     4422      +10     
+ Misses        200      199       -1     
  Partials       26       26              

Flag	Coverage Δ
CI-GHA	95.11% <100.00%> (+0.03%)	⬆️
MyPy	41.10% <72.72%> (+<0.01%)	⬆️
OS-Linux	99.38% <100.00%> (+<0.01%)	⬆️
OS-Windows	99.48% <100.00%> (+<0.01%)	⬆️
OS-macOS	99.08% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	98.97% <100.00%> (+<0.01%)	⬆️
Py-3.10.14	99.22% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	99.22% <100.00%> (+<0.01%)	⬆️
Py-3.12.5	99.22% <100.00%> (+<0.01%)	⬆️
Py-3.13.0-rc.1	99.22% <100.00%> (+<0.01%)	⬆️
Py-3.8.10	98.92% <100.00%> (+<0.01%)	⬆️
Py-3.8.18	99.16% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	98.92% <100.00%> (+<0.01%)	⬆️
Py-3.9.19	99.16% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.11	99.22% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	99.22% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.17	99.25% <100.00%> (+<0.01%)	⬆️
VM-macos-latest	99.08% <100.00%> (+<0.01%)	⬆️
VM-ubuntu-latest	99.38% <100.00%> (+<0.01%)	⬆️
VM-windows-latest	99.48% <100.00%> (+<0.01%)	⬆️
pytest	99.38% <100.00%> (+<0.01%)	⬆️

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