[py] Refactored Proxy object in common/proxy.py using custom descriptor.

[sandeepsuryaprasad]

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Description

Moved all below ProxyTypes which has common operations in getter and setter methods to one single descriptor class. Also, I have re-factored to_capabilities method by eliminating multiple conditionals.

autodetect 
ftpProxy
httpProxy
noProxy
proxyAutoconfigUrl
sslProxy
socksProxy
socksUsername
socksPassword
socksVersion

This is the same PR (12280) that I closed as I had some issues with the branch that I was working with.
In this PR, I have incorporated the change that @AutomatedTester had asked. (giving blank lines between docstring and descriptor object)

Motivation and Context

• By Moving all ProxyType getters and setters to custom descriptors, multiple properties are being eliminated.
• Currently there are 10 pairs of property with 20 methods. All doing similar job of validating the proxy type and assigning a class attribute with some proxy, there is lot of code repetition.
• This is now completely optimised by moving all properties to descriptor class.
• This reduces maintenance effort since the entire logic of attribute assignment lies in __set__ method of descriptor.
• Also, refactored to_capabilites method in Proxy class by eliminating multiple conditional statements.

Types of changes

• [] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• [] My change requires a change to the documentation.
• [] I have updated the documentation accordingly.
• [] I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Patch coverage: 2.70% and project coverage change: +0.37 🎉

Comparison is base (50f9252) 56.92% compared to head (b543937) 57.29%.

❗ Current head b543937 differs from pull request most recent head eff7ec3. Consider uploading reports for the commit eff7ec3 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #12286      +/-   ##
==========================================
+ Coverage   56.92%   57.29%   +0.37%     
==========================================
  Files          86       86              
  Lines        5423     5367      -56     
  Branches      193      193              
==========================================
- Hits         3087     3075      -12     
+ Misses       2143     2099      -44     
  Partials      193      193              

Impacted Files	Coverage Δ
py/selenium/webdriver/common/proxy.py	15.09% <2.70%> (-2.19%)	⬇️

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