feat: Proxy server header capability

[mohitanand-cred]

Changes:

• Added support for custom header and query items parameter.
• New struct called ProxyServerConfig which takes in proxy server url and a delegate(MixpanelProxyServerDelegate).
• MixpanelProxyServerDelegate can be used to provide headers or query items to the proxy server. The delegate func mixpanelResourceForProxyServer(_ mixpanel: MixpanelInstance) -> ServerProxyResource? is being called when its time flush the batch request.
• Created a new initialize function in Mixpanel which takes in ProxyServerConfig instead of just serverURL.