fix(proxy): include original request signal in proxy request

[BarryThePenguin]

This includes the original Request#signal in the RequestInit options use to create the proxy request

Currently Cloudflare doesn't expose the abort event to Workers, but it looks like there is work to implement it, see:

• Feature proposal: Signal client disconnect using Request.signal cloudflare/workerd#3033
• Implement Request.signal to detect client disconnects cloudflare/workerd#3488

I'm unsure about other platforms..

fixes #3924

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.32%. Comparing base (0c405b9) to head (00e95cd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3925   +/-   ##
=======================================
  Coverage   91.32%   91.32%           
=======================================
  Files         168      168           
  Lines       10689    10690    +1     
  Branches     3056     3019   -37     
=======================================
+ Hits         9762     9763    +1     
  Misses        926      926           
  Partials        1        1           

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[usualoma]

Hi @BarryThePenguin. Thank you. LGTM!

[yusukebe]

LGTM!

[yusukebe]

@BarryThePenguin

Thank you! Merging.