SEP-9: add proof_of_income field

[JakeUrban]

What problem does your feature solve?

Anchors often have different KYC "tiers" that allow their users to transact in small to large amounts. For example, with a limit of $2k per day, anchor could require a user's contact information and ID number, but if the user wants to transact more than $2k per day, then they have to provide additional KYC documentation.

However, the SEP-9 list of standard KYC fields does not include a field used for collecting income information.

What would you like to see?

Adding a proof_of_income field of type binary standardizes anchors' request for the user to provide documentation proving their income level. Anchors may need this information to upgrade their users' to a higher "tier".

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.

[JakeUrban]

cc @jessieserrino @howardtw

[JakeUrban]

@CassioMG

[github-actions]

This issue is stale because it has been open for 30 days with no activity. It will be closed in 30 days unless the stale label is removed.

[CassioMG]

@JakeUrban this looks great to me. Would you mind if I open a PR for this change?

[JakeUrban]

Please do!

[CassioMG]

Here it is: #1310