SEP-12 & SEP-09 improvements

SEP-12:

• fix: referral_id is now of type string. It was data before.
• add: the possibility to provide extra (custom) fields and files when adding/updating a customer data
• improve sorting of fields and files in the the multipart request to put customer data: files are now attached to the end of the multipart request
• fix: provided_fields parsing form the get customer info response

SEP-09:

• add new fields: mobile_number_format, external_transfer_memo, bank_name, mobile_money_number and mobile_money_provider
• add card fields