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.

The default handler, if no handler is specified by the user, will simply return the IP attached to the request object.

An additional handler should be available within the library, which looks into the X-Forwarded-For header and gets the first IP in the list if it exists, falling back to the IP attached to the request if the header isn't available. This is an implementation that users can optionally use instead of making their own.

[UmanShahzad]

Released in v2.4.0.