sso_proxy: use ProxyFromEnvironment in http Transport

[saithejareddy]

Problem

As described in #305, the HTTP client used in sso_proxy doesn't allow for use of HTTP_PROXY environment variables -- meaning traffic can't be passed through a proxy if desired.

Solution

Make use of the http.ProxyFromEnvironment method, which allows for usage of these environment variables.
This is not added in http_client.go
https://github.com/buzzfeed/sso/blob/main/internal/proxy/providers/http_client.go#L11

[codecov]

Codecov Report

Merging #310 (244da51) into jusshersmith-auth-ProxyFromEnvironment (63e450a) will increase coverage by 0.89%.
The diff coverage is 83.85%.

@@                            Coverage Diff                             @@
##           jusshersmith-auth-ProxyFromEnvironment     #310      +/-   ##
==========================================================================
+ Coverage                                   61.84%   62.73%   +0.89%     
==========================================================================
  Files                                          57       58       +1     
  Lines                                        4188     4286      +98     
==========================================================================
+ Hits                                         2590     2689      +99     
+ Misses                                       1388     1382       -6     
- Partials                                      210      215       +5     

Impacted Files	Coverage Δ
internal/proxy/logging_handler.go	16.32% <0.00%> (ø)
internal/proxy/proxy.go	16.00% <0.00%> (-2.61%)	⬇️
internal/proxy/proxy_config.go	76.21% <ø> (ø)
internal/proxy/options.go	82.07% <85.00%> (-1.52%)	⬇️
internal/proxy/configuration.go	89.80% <89.80%> (ø)
internal/proxy/metrics.go	86.20% <100.00%> (ø)
internal/proxy/oauthproxy.go	54.26% <100.00%> (+0.47%)	⬆️

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