feat: support proxy

[monkeyWie]

The proxy will work for several case:

• HTTP resolve and download
• BitTorrent trackers (http,udp,websocket)
• Extension install
• Requests in extension script engine

[codecov-commenter]

Codecov Report

Attention: 38 lines in your changes are missing coverage. Please review.

Comparison is base (ee7da17) 60.31% compared to head (f040323) 60.11%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
pkg/rest/server.go	33.33%	16 Missing and 2 partials ⚠️
pkg/util/url.go	0.00%	10 Missing ⚠️
pkg/download/extension.go	61.53%	4 Missing and 1 partial ⚠️
internal/protocol/bt/fetcher.go	0.00%	2 Missing and 1 partial ⚠️
pkg/download/downloader.go	50.00%	1 Missing ⚠️
pkg/download/engine/engine.go	85.71%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
- Coverage   60.31%   60.11%   -0.20%     
==========================================
  Files          19       19              
  Lines        2749     2803      +54     
==========================================
+ Hits         1658     1685      +27     
- Misses        896      920      +24     
- Partials      195      198       +3     

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