How to integrate custom schemaUri for SDKs

[rjalander]

The schemaUri defined in latest spec 0.4.0, needs to be integrated with SDKs

"schemaUri": {
    "type": "string",
    "minLength": 1,
    "format": "uri"
}

[afrittoli]

This PR shall provide some guidance about this issue: cdevents/sdk-go#87.

The SDK shall be able to validate events against the provided schemaUri, with the following notes:

• validation on the CDEvents schema must be verified first
• the SDK MUST provide an API for users to load the schema from a string or []byte and associated it with the schemaUri, so that lookup of the schema is done in memory rather than over the internet
• the SDK MAY provide an option to attempt and download the schema from the schemaUri directly via HTTPS
• the SDK MUST fail validation if HTTPS download is not available/disabled and the schema is not in memory