Add suport for HTTP_PROXY/HTTPS_PROXY variables

[koles]

Motivation

• API access from restricted environments

Summary

• Add optional proxy parameter to GoodDataApiClient and GoodDataSdk.create()
• Auto-detect proxy from HTTPS_PROXY / https_proxy / HTTP_PROXY / http_proxy environment variables when no explicit proxy is provided
• Explicit proxy parameter takes precedence over environment variables

Test plan

• Unit tests for explicit proxy parameter
• Unit tests for env variable detection (all four variants)
• Unit tests for precedence (HTTPS_PROXY > HTTP_PROXY, explicit > env)
• Unit tests for no-proxy case
• Integration through GoodDataSdk.create()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.83%. Comparing base (efc6b2a) to head (bb667f9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1587   +/-   ##
=======================================
  Coverage   78.83%   78.83%           
=======================================
  Files         230      230           
  Lines       15486    15491    +5     
=======================================
+ Hits        12208    12213    +5     
  Misses       3278     3278           

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