Add generic ability to add extension key-value for Errors

[phillip-kruger]

Relate to #282. We have code as a key supported with #295 but still need a generic key value.

Some more features:

• Move the knowledge to the model, so that we do not need to check annotations in runtime.
• Allow adding of Annotation on method that throw the exception, to get per instance usage (currently it's global)
• Add way to do all that is done for code for any other key-value

[t1]

I think we should add an instance extension, too; probably an UUID. This would be logged on the server, so when the client reports an error, it's easy to find the exact request without relying on the timestamp or so. In rfc-7807, it's an URI, so it could be an URN like urn:uuid:1234-1232534-... or an URL directly into the logging system.

[piotrblasiak]

I think graphql-java has the most flexible, if not the most developer friendly approach. There you register a DataFetcherExceptionHandler and override getExtensions in the thrown GraphQLError. This makes it possible to add 3 important use cases for exception extensions:

1. (Static) per class - like the @errorcode already does
2. Instance - additional details that do not fit into the message field of an error but that are still relevant and specific to an exception instance.
3. Handled throw - like @t1 suggests an UUID or other kind of reference, whatever the developer wants to call it. This probably needs to be added to the exception after it has been thrown.