SEP31: Added SEP-12 logic

[JakeUrban]

resolves stellar/integration-meta#147

Added the ability to use SEP-12 for KYC'ing senders and receivers of SEP-31 transactions.

• Changes the format of the GET /info response to include sender_sep12_type and receiver_sep12_type
• Removed fields other than memo in the sender and receiver objects for GET /info and POST /send
• Changed the customer_info_needed error response to signal a required update to the SEP-12 PUT /customer endpoint
• Renamed the original customer_info_needed error response to transaction_info_needed
• Changed pending_info_update Transaction status to pending_transaction_info_update
• Added pending_customer_info_update Transaction status for updates to SEP-12 PUT /customer

[JakeUrban]

This isn't ready yet. We'll likely wait for the SEP-12 changes to land before merging this, and there are still some adjustments we're considering.

[msfeldstein]

lgtm