improved way of disable datasets

[gwdawson]

currently if you want to disable datasets you need to do the following:

1. pass the enableDatasets = false prop into SqlQueryEditor.
2. provide a custom toRawSql function, that doesn't expect a dataset

this pr removes the need for code duplication in all datasources with disabled datasets by no longer requiring a custom toRawSql function.

this pr also also changes where you declare disableDatasets, this is now declared in datasource.ts opposed to previously this was just a prop in the builder component.

---

after this pr is merged, the only thing you need to do is add the flag disableDatasets: true to the getDB function inside of datasource.ts.

getDB(): DB {
  return {
    {... your other database options}
    disableDatasets: true, // <---- just add this line here
  };
}

[gabor]

LGTM 👍

[gwdawson]

@grafana/enterprise-datasources can i please get a review from your side also, nothing needs to change in your datasources as the default behaviour is still to "use datasets"