function/endpoint for leads

[nazreen]

the schemas was implemented here: #108

what we need now are the endpoints. we can just model it after the other entities' endpoint handlers such as in this file:
https://github.com/OpenPromisesProject/op-serverless-api/tree/master/functions/api

notes:

• this endpoint will be used later either by a messenger webhook or by a ui form (to be confirmed)