SNOW-1333163 support for default proxy

[sfc-gh-knozderko]

Description

SNOW-1333163 Add support for default proxy.

When specifying allowEmptyProxy=true and useProxy=true but without specifying the value of proxyHost, the proxy would be enabled but not configured allowing to use a default proxy provided by the operating system.

Checklist

• Code compiles correctly
• Code is formatted according to Coding Conventions
• Created tests which fail without the change (if possible)
• All tests passing (dotnet test)
• Extended the README / documentation, if necessary
• Provide JIRA issue id (if possible) or GitHub issue id in PR name

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ (sfc-gh-knozderko)[https://github.com/sfc-gh-knozderko]
❌ @VincentCroquette
_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.67%. Comparing base (982a1e4) to head (737d695).

Files	Patch %	Lines
Snowflake.Data/Core/Session/SFSessionProperty.cs	85.71%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #997      +/-   ##
==========================================
+ Coverage   85.55%   85.67%   +0.12%     
==========================================
  Files         106      106              
  Lines       10777    10809      +32     
  Branches     1032     1037       +5     
==========================================
+ Hits         9220     9261      +41     
+ Misses       1307     1299       -8     
+ Partials      250      249       -1     

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[vc-74]

I have read the CLA Document and I hereby sign the CLA

[sfc-gh-dstempniak]

LGTM