feat: Add Missing User Identities

[BrandonStalnaker]

Background

• A client using the Go SDK to forward event data to mParticle asked whether mobile number is supported. UserIdentities was missing mobile_number (and the related phone identity fields), even though those identities already existed in IdentityType and in other SDKs such as the Apple SDK.

What Has Changed

• Added MobileNumber, PhoneNumber2, and PhoneNumber3 to UserIdentities so events can include those identity values.
• Added a unit test covering JSON marshaling for the new phone number identity fields.

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://go/j/[ticket-number]

[cursor]

PR Summary

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