registration: Support custom classes in "ResourceModules" schema

registration: Support custom classes in "ResourceModules" schema

Instead of expecting everything to be a ResourceLoaderFileModule, use
the "anyOf" property to validate against multiple schemas. The following
schemas are now allowed:
* The ResourceLoaderFileModule schema, now with the "targets" property
* A schema for ResourceLoaderImageModule
* A generic schema that requires the "class" property is set, but not to
"ResourceLoaderFileModule" or "ResourceLoaderImageModule".

The last schema will allow for any custom ResourceLoaderModule class to
be set in extension.json with arbitrary parameters.

One downside of this is that the error messages shown when a file does
not validate get a little more confusing, as it shows the error messages
for each schema it does not match.

Bug: T105236
Change-Id: I5e4bfa69c733187c7b27294c159cac05b3b92e81