sso_*: use ProxyFromEnvironment in http Transport

[Jusshersmith]

Problem

As described in #305, some HTTP clients used in sso_auth and sso_proxy don'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.

We already make use of this in one HTTP client within sso_proxy here:

sso/internal/proxy/reverse_proxy.go

Line 53 in 03e1259

Proxy: http.ProxyFromEnvironment,

[codecov]

Codecov Report

Merging #307 (26d56b0) into main (290f27e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #307   +/-   ##
=======================================
  Coverage   62.73%   62.73%           
=======================================
  Files          58       58           
  Lines        4286     4286           
=======================================
  Hits         2689     2689           
  Misses       1382     1382           
  Partials      215      215           

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