Relay custom customer traits to Friendbuy destination

[dondi]

What does this PR do?

This PR allows users of the Friendbuy destination to relay customized customer properties to Friendbuy, beyond the standard set that is currently accepted.

Are there breaking changes in this PR?

No.

Any background context you want to provide?

Friendbuy allows its merchants to add custom traits to their customers when integrating directly with the Friendbuy customer tracking API as part of a feature called widget targeting: http://help.friendbuy.com/en/articles/1916126-widget-targeting

We would like to allow our merchants to pass these custom parameters through the identify event to Friendbuy's Destination integration, so that they can use this feature via Segment. The Friendbuy Segment integration currently only accepts Customer ID, Email Address, First Name, and Last Name. We'd like to be able to allow merchants to pass in custom parameters through the identify event, such as age, VIP, etc.

Is there parity with the server-side/android/iOS integration components (if applicable)?

n/a

Does this require a new integration setting? If so, please explain how the new setting works

No.

Links to helpful docs and other external resources

General customer tracking API: https://developers.friendbuy.com/#tracking

Widget targeting feature: http://help.friendbuy.com/en/articles/1916126-widget-targeting