Support proxy env for extension downloads

[adsharma]

Fixes: #415

Summary

• read Ladybug-specific and standard proxy environment variables for extension downloads
• apply the resolved proxy settings to httplib clients used for extension libraries and installers
• document the supported variables and add focused proxy parsing/env selection tests

Root Cause

Ladybug used httplib directly for extension downloads, but did not pass proxy settings into the client. httplib supports proxies through set_proxy, but it does not automatically consume process proxy environment variables.

Validation

• cmake --build build/release --target types_test
• build/release/test/common/types_test '--gtest_filter=ExtensionProxyTest.*'
• git diff --check