Add a schema uri property to the context

[e-backmark-ericsson]

To simplify for event consumer we should include a schema uri property to the cdevents context object. With that the consumer could use that schema for validation, parsing and data extraction purposes. It might be obvious that an event sent using a certain version of the spec would be possible to retrieve from https://cdevents.dev//schema/.json, but there are use cases where that might not be enough:

• If the event sender is not allowed to communicate with sites on Internet and would instead point at an internally managed schema registry serving the event schemas
• If cdevents.dev is not seen as reliable enough when it comes to performance and availability, and thus a reference to a more robust schema registry could be provided
• If the organization wants to limit the value range or optional parameters provided in the events and thus will provide references to a different schema registry than cdevents.dev to show what actual schemas where used when producing the event sent. It could for example limit some values in test suite/case events differently depending on what tool is used in a specific test activity.