COVID 19 - European C# API to verify Covid Pass tests and vaccinations

This is so far just proof of concept, but with little work, this can be amazing API.

A lot of work was based on EU documentation: https://github.com/ehn-dcc-development/ehn-dcc-schema/blob/release/1.3.0/DCC.combined-schema.json

And on this amazing project in TypeScript: https://github.com/covidpass-org/covidpass