TT-12174 Added support for proxy settings loaded from environment variables

Added http.ProxyFromEnvironment to creation of transport for the default client.

Description

In the main.go file the transport of the http.DefaultClient is overwritten because InsecureSkipVerify: config.SSLInsecureSkipVerify is added. This disables the default behavior of go that the environment variables HTTP_PROXY, HTTPS_PROXY and NO_PROXY are used for HTTP connections.

Motivation and Context

The change allows to use the tyk identity broker to be used in environments where access to external services is managed by a http porxy.

How This Has Been Tested

Patched image used to gain access to a identiy provider via HTTP Proxy

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Refactoring or add test (improvements in base code or adds test coverage to functionality)

Checklist

[x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side). If pulling from your own fork, don't request your master!
[x] Make sure you are making a pull request against the master branch (left side). Also, you should start your branch off our latest master.
[ ] My change requires a change to the documentation.
- [ ] If you've changed APIs, describe what needs to be updated in the documentation.
- [ ] If new config option added, ensure that it can be set via ENV variable
[ ] I have updated the documentation accordingly.
[ ] Modules and vendor dependencies have been updated; run go mod tidy && go mod vendor
[ ] When updating library version must provide reason/explanation for this update.
[ ] I have added tests to cover my changes.
[x] All new and existing tests passed.
[x] Check your code additions will not fail linting checks:
- [x] go fmt -s
- [x] go vet

TykTechnologies / tyk-identity-broker