feat(discovery-v2): update models and add several new methods

New methods are listDocuments, getDocument, listDocumentClassifiers, createDocumentClassifier,
getDocumentClassifier, updateDocumentClassifier, deleteDocumentClassifier,
listDocumentClassifierModels, createDocumentClassifierModels, getDocumentClassifierModels,
updateDocumentClassifierModels, deleteDocumentClassifierModels, getStopwordList, createStopwordList,
deleteStopwordList, listExpansions, createExpansions, deleteExpansions

Author: Hsaylor