Centralize the transaction building and transaction validation logic into a package

[littlejohntj]

Because of the AWS migration, it was easier to make progress by moving the transaction building code into the transaction-request serverless app.

We should move back to a package once we figure out the correct setting for turborepo for packages in our set up. This will make the transaction building code more portable and reusable as well as increase testing. It'll also by help simplifying the transaction request server.