Add support to private network CORS

[alessio-perugini]

SSIA

[rhpco]

LGTM!

[umbynos]

Indeed, it fixes the problem:

$ curl -X OPTIONS 'http://127.0.0.1:8991/pause' -H 'Origin: https://app.arduino.
cc' -H 'AllowPrivateNetwork: true' -v
*   Trying 127.0.0.1:8991...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 8991 (#0)
> OPTIONS /pause HTTP/1.1
> Host: 127.0.0.1:8991
> User-Agent: curl/7.68.0
> Accept: */*
> Origin: https://app.arduino.cc
> AllowPrivateNetwork: true
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 204 No Content
< Access-Control-Allow-Credentials: true
< Access-Control-Allow-Headers: Origin,Authorization,Content-Type
< Access-Control-Allow-Methods: PUT,GET,POST,DELETE
< Access-Control-Allow-Origin: https://app.arduino.cc
< Access-Control-Allow-Private-Network: true
< Access-Control-Max-Age: 50
< Vary: Origin
< Vary: Access-Control-Request-Method
< Vary: Access-Control-Request-Headers
< Date: Mon, 18 Dec 2023 16:13:29 GMT
< 
* Connection #0 to host 127.0.0.1 left intact

I've also tried with:
curl -X POST 'http://127.0.0.1:8991/pause' -H 'Origin: https://test.arduino.cc' -v and it gives a 403. Whereas with the previous implementation, it was returning a 200.. In either cases it seems that with the wrong origin the request is not processed at all. I don't think that's a problem.

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (fb135b0) 20.94% compared to head (e55c67e) 20.93%.

Files	Patch %	Lines
main.go	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #873      +/-   ##
==========================================
- Coverage   20.94%   20.93%   -0.02%     
==========================================
  Files          43       43              
  Lines        3151     3153       +2     
==========================================
  Hits          660      660              
- Misses       2395     2397       +2     
  Partials       96       96              

Flag	Coverage Δ
unit	20.93% <0.00%> (-0.02%)	⬇️

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