SEP-9: add proof_of_liveness field

[theclabs]

What problem does your feature solve?

As an extension of #1301 Anchors often have different KYC "tiers" that allow their users to transact in small to large amounts.
To increase these levels, Anchors usually ask for proof of liveness, either as a selfie or videos with challenges, as well as requesting photo_proof_residence or proof_of_income.

However, the SEP-9 list of standard KYC fields does not include a field used to do a user proof of liveness.

What would you like to see?

Adding a proof_of_liveness field of type binary standardizes anchors' request for the user to provide dynamic images or videos proving their liveness. Anchors may need this information to upgrade their users' to a higher "tier" or revalidate a expired KYC.

{
    "status": "NEEDS_INFO",
    "fields": {
        "proof_of_liveness": {
            "description": "A clear photo of the user face with no glasses",
            "type": "binary"
            }
    }
}

or

{
    "status": "NEEDS_INFO",
    "fields": {
        "proof_of_liveness": {
            "description": "A clear video of the user face smiling",
            "type": "binary"
            }
    }
}

This field could be passed in a SEP-12 PUT /customer OR POST /customer/files request. If passed to POST /customer/files, the file ID returned would then need to be referenced in the subsequent PUT /customer request.

[github-actions]

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