Mock API - business implementation

[onthebreeze]

APi specification has been updated https://jargon.sh/redoc.html?url=/user/unece/traceabilityEvents/v/working/artefacts/openapi/render.json. Notable changes are

• to remove the traceability graph structure and replace with a simple GET collection on the Events abstract type (implemented as Open API Any-of the four concrete event types.). Also added filter parameters and pagination to this API

• please implement all methods in a simple stateful mock that allows POST/PUT/GET of individual events and the GET collection query over any posted events that match filter conditions. The GET collection will, in future, need to be more carefully scoped to avoid returning sensitive data.