Feature Request: Implement API and CLI

[emmambd]

In order to further scale the GBFS validator's efforts and regularly run validation on GBFS feeds in systems.csv, MobilityData is interested in supporting the extension and maintenance of Entur's GBFS validator.

Before the GBFS validator can be used by MobilityData, it needs to include:

• An API, with an open schema
• cli tool

Related issues

• #83
• #97

[testower]

I have now converted this project to maven multi-module in preparation for new api and cli modules.