Add HTTP proxy support to MDCB

[jonathanfoster]

It would be great if MDCB connections could be proxied through HTTP proxies. This would eliminate the need to update firewall rules in restricted environments.

Currently, I'm in a restricted corporate environment that doesn't allow outbound connections without either using the corporate HTTP proxy or requesting egress firewall rules be added. And if you're a Tyk Cloud customer, there's currently no reasonable range of IPs to whitelist for Tyk Cloud MDCB instances other than the entire range of IPs from AWS.