TT-12072 Configure Splunk client to use proxy from environment

[jonathanfoster]

This PR resolves issue #816 by configuring the Splunk client to use proxy settings from the environment.

Description

I set the proxy to http.ProxyFromEnvironment when initializing the Splunk client transport.

Related Issue

• TT-12057 Splunk pump ignores HTTP proxy environment variables #816

Motivation and Context

Support for HTTP_PROXY environment variables is required when running Tyk Pump in restricted environments that require all HTTP traffic to go through a proxy.

How This Has Been Tested

I included a unit test that verifies the proxy is not nil after initializing a Splunk client. I attempted to fully test the functionality by setting the HTTP_PROXY environment variable, but this caused other Splunk tests to fail. Since this fix leverages existing functionality from http.ProxyFromEnvironment, I don't think it's necessary to have end-to-end test coverage.

Technically one could set Proxy to something other than ProxyFromEnvironment and the test would pass so this is a gap. It would be ideal to verify the function pointers are the same, but I couldn't get it to work. I'm open to feedback on how to better handle this test.

Screenshots (if appropriate)

Not applicable.

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)

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