Add optional IP selection handler

[UmanShahzad]

Add an optional IP selection handler to the SDK client initialization step which accepts the request context and expects returning an IP.

Add a default handler for this which looks at the X-Forwarded-For header and falls back to the source IP.

The resulting IP is the IP for which details are fetched.

[UmanShahzad]

Closing as this actually only applies to middleware SDKs (e.g. Django/Rails/Express/etc), not language SDKs.