Feature Request: external_id

[makinghappen]

Checklist before submitting a feature request

• I've updated to the latest released version of the SDK
• I've searched for existing Github issues
• I've read the Code of Conduct

Goals

Library does not support passing external_id used for advanced matching
Can it be implemented?

ios library already has it
facebook/facebook-ios-sdk#1496

Expected results

support external_id

Code samples & details

// INSERT YOUR CODE HERE

[Solracjg]

#1208