Adds PUT method to OpenAPI spec.
Given that the transaction IDs are generated on server-side, for the PUT
method to remain idempotent, it can only update existing transactions.
It also adds a TransactionExists method on the DAL.
Issue: #20
It adds the method to the OpenAPI spec and generates a new server
config. The requirement for the ID on the Transaction component is
removed, so that it can be reused for insertions.
It also adds two new middlewares, a logging and a spec validator. If a
request does not follow the spec, a 400 is returned immediately.
Issue: #18
