Added support for env *_PROXY variables for agent loadbalancer

[pierre-az]

Types of Changes

To me, this is a bugfix as the doc specifies we can use these env variables for k3s: https://docs.k3s.io/advanced#configuring-an-http-proxy.
There is also this documentation that may be misleading as it suggests that in addition to proxies, you should open your network to various URLs, which is incompatible: https://docs.k3s.io/installation/requirements#networking.

Verification

You can test the new agent on a airgapped environment, where internet access is only possible through an external proxy.

This was tested in production on such environment on my end.

Testing

There are no linked unit tests as this is the first time I'm working on Go.

Linked Issues

Linked issue is: #8976

User-Facing Change

HTTP_PROXY, HTTPS_PROXY and NO_PROXY environment variables are now taken into account by the agent loadbalancer if K3S_AGENT_HTTP_PROXY_ALLOWED env variable is set to true.
This however doesn't affect local requests as the function used prevents that: https://pkg.go.dev/net/http#ProxyFromEnvironment. 

Further Comments

As this is a known issue for net.Dialer, this fix is based on another library which can be found on: https://github.com/mwitkow/go-http-dialer

[pierre-az]

After destroying accidentally #8982, here's the new PR with requested changes, really sorry about the mess.

[codecov]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (7101af3) 46.24% compared to head (f3e29dd) 41.59%.

Files	Patch %	Lines
pkg/agent/loadbalancer/servers.go	32.35%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9070      +/-   ##
==========================================
- Coverage   46.24%   41.59%   -4.66%     
==========================================
  Files         148      148              
  Lines       15924    15954      +30     
==========================================
- Hits         7364     6636     -728     
- Misses       7374     8185     +811     
+ Partials     1186     1133      -53     

Flag	Coverage Δ
e2etests	?
inttests	38.81% <32.35%> (-0.07%)	⬇️
unittests	18.35% <32.35%> (+0.01%)	⬆️

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