Added option to use Proxy

[lordgreg]

This is the initial Pull Request, created in nokia/kong-oidc by troll (PR).

This PR adds ability to use http/https Proxy.

The use-case could be if you are behind corporate proxy, thus discovery not being accessible. If http_proxy and https_proxy variables are being used, then you should also see next lines in logs:

[debug] 2368#0: *6148 [lua] openidc.lua:430: openidc_configure_proxy(): openidc_configure_proxy : use http proxy

instead of

[debug] 2368#0: *6148 [lua] openidc.lua:430: openidc_configure_proxy(): openidc_configure_proxy : don't use http proxy

👍🏻

[lordgreg]

@cristichiru are you able to merge this PR or is there still anything else I have to do?

Thank you :)