Support *_verification fields in PUT /customer requests

[JakeUrban]

What problem does your feature solve?

SEP-12 includes a PUT /customer/verification endpoint that is meant to accept verification of fields previously provided to PUT /customer, like mobile_number or email_address. However, the anchor platform doesn't support this endpoint, and it doesn't support accepting the mobile_number_verification or email_address_verification fields in PUT /customer requests.

What would you like to see?

Instead of supporting PUT /customer/verification, allow the anchor to accept *_verification fields in PUT /customer, where * is a valid SEP-9 field. This will allow anchors implement a 2-step process where they first accept the field that requires verification and then request the verification of that field.

What alternatives are there?

The alternative is to support the flow described in SEP-12, which is to set the field requiring verification to the VERIFICATION_REQUIRED status in GET /customer responses, and accept the verification with the PUT /customer/verification endpoint.

I don't think this is the best approach because we are trying to move away from using GET /customer as a means of determining which fields need to be provided to the anchor, in favor of the client polling the transaction record for which KYC may be required instead.

cc @Ifropc

[Ifropc]

Added proposal to simplify verification process using only PUT /customer endpoint: stellar/stellar-protocol#1431

[Ifropc]

Tracked internally as ANCHOR-582