Handle registration of schemas with the same $id

[kiendang]

Raise a value error when attempting to register a schema with the same $id and version with another one that has already been registered.

Also allow register_schema_file to accept a file object as input. Would be useful for loading client schemas imported using importlib.resources which are returned as file objects.

outdated stuff below

This add options how to handle registering a schema having the same `$id` with another schema that has already been registered.

Currently if the input schema has the same $id and version as an existing schema, it would just override it. One situation where this is not ideal is the case of client events in jupyter-server/jupyter_server#364. A client can override and mimic a server event just by having the same $id and version.

This PR add a new argument duplicate to register_schema_file that takes one of the following values

'raise': raise a ValueError when registering a schema having the same $id with another schema that has already been registered

'allow': If the input schema has the same $id and version as another schema that has already been registered, it would override the existing schema. In case the input schema only has the same $id as an existing schema but a different version, the input schema would be registered along side the existing schema.

'skip': does nothing

default is set to be 'raise' which is the strictest option.

[Zsailer]

LGTM. Thanks, @kiendang!