Promise based makeExecutableSchema service method

[valeeum]

The ability to override makeExecutableSchema in the service is a great feature that opens up possibilities of doing interesting stuff like schema stitching, merging, etc. However, it would be much more useful if it allowed the response to be promise based.

			/**
			 * Call the `makeExecutableSchema`. If you would like
			 * to manipulate the concatenated typeDefs, or the generated schema,
			 * just overwrite it in your service file.
			 * @param {Object} schemaDef
			 */
			makeExecutableSchema(schemaDef) {
				return makeExecutableSchema(schemaDef);
			},