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

[codingWombat]

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.

Related Issue

#393

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

• 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

• 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!
• 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.
• All new and existing tests passed.
• Check your code additions will not fail linting checks:
  • go fmt -s
  • go vet