Add billing phone verification inputs

[oschwald]

Adds three optional inputs to withBilling(), as array keys or named arguments: phone_verification_method (delivered_code, network, other), phone_was_verification_successful (boolean), and phone_verification_time (RFC 3339 date-time). They describe the most recent verification of the billing phone number. When input validation is on, the client checks the method value, the boolean type and the time format. The first commit moves the RFC 3339 check into one helper that the event time also uses.
Reviewer question: the new named arguments are in alphabetical order, between $phoneNumber and $postal, as the tracking_token change did. A caller who passes $postal or $region by position would shift into the new arguments. Should they go at the end instead?

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