* added $schema field support in plugin.schema.json

More details can be found in https://github.com/grafana/plugin-validator/issues/1

* Others category in plugin.json, minimum 1 keyword 

* Allow others as a category if the plugin doesn't fall into appropriate category. Refer https://github.com/grafana/plugin-validator/issues/3 for more details
* Mandate minimum one keyword for better discovery in plugin repository.

* Dropped enum from plugin.json $schema