feat: gemini + proxy

[iorisa]

Features

• Add proxy to gemini LLM. Ref: https://zhuanlan.zhihu.com/p/679115466
• Fixes Issue with Proxy Configuration in config2.yaml Not Working in MetaGPT v0.7.3--在MetaGPT v0.7.3中使用config2.yaml配置代理无效，但通过os包设置环境变量可以 #958

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.20%. Comparing base (26252e6) to head (f6260ec).
Report is 9 commits behind head on v0.7-release.

Files with missing lines	Patch %	Lines
metagpt/provider/google_gemini_api.py	40.00%	3 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@              Coverage Diff              @@
##           v0.7-release     #971   +/-   ##
=============================================
  Coverage         81.19%   81.20%           
=============================================
  Files               226      226           
  Lines             12477    12482    +5     
=============================================
+ Hits              10131    10136    +5     
  Misses             2346     2346           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[stellaHSR]

LGTM