Swap direct access to the DB on the API server with an data abstraction
layer.
Implement each API type converter separately and revert changes to the
auto-generated server implementation types.
Add error return to DAL methods. Implement `Transactions`.
Add tools.go with oapi-codegen and mockgen.
https://www.jvt.me/posts/2022/06/15/go-tools-dependency-management/
Update go packages.
Issues #5, #12
Introduces the api-server/main.go, which creates the echo server,
connects it to the implemented handlers and runs it.
Fix the SQL query to retrieved transactions w/ categories.
Issues #6 and #7
