loader: change extension handler type (#6015)

This

1. changes the extension.Handler type to make it more flexible
2. simplifies the extension usage -- it used to be called in many places,
   but it could all be handled through util.Unmarshal and util.UnmarshalJSON
   instead

We've previously marked it as "EXPERIMENTAL", so we should have enough
leeway to change this now.

NOTE: As a consequence of (2.), we're no longer accepting trailing data for
json files loaded with OPA. I believe it wasn't intentional to ignore bad data
before -- now, it'll be an error.

Signed-off-by: Stephan Renatus <stephan@styra.com>