Add support for HTTP proxy configuration

[olivielpeau]

What does this PR do?

Adds support for HTTP proxy configuration, with consistent behavior with the proxy setting of the HTTP output plugin (https://docs.fluentd.org/output/http#proxy) such as defaulting to standard HTTP_PROXY env vars, except that the option is called http_proxy for our plugin.

Motivation

Customer request

Additional Notes

Related docs: proxy config on the persistent http client we use: https://www.rubydoc.info/gems/net-http-persistent/3.1.0/Net/HTTP/Persistent#initialize-instance_method

Testing:

• I've run the tests locally, they pass.
• this change was confirmed to work on a lab setup by a Solutions Engineer