[SIP-5] QueryBuilder in the client for granularity and groupby in word cloud (#6377)

* [SIP-5] QueryBuilder in the client for groupby field

- Lay the structure of the QueryContext builder in the client
- QueryContext builder composes different portions of the queryContext object (to be sent to server from the client) from the datasourceBuilder and the queryObjectBuilder.
- The datasourceBuilder builds the datasource id and type by parsing them from the datasource field in formdata
- The queryObjectBuilder currently only builds the groupby field. It will further compose the queryObject from sub query builders in future PRs.
- Create a buildQuery method for WordCloud and override the groupby value with the value of series from formdata.

* Addressing PR comments
- Rename query builder files and their default exports
- Move tests into spec/javascripts/superset-ui for easy mass migration to superset-uiin the future
- Define viz specific formData and export formData for each viz type as intersection type of the generic formData and the viz specific one
- Specify the type signature for sqla and druid based data sources

* Addressing additional PR comments.
- Introduce a Datasource class and leverage Typescript's declaration merging of the interface
by the same name.
- Let Typescript infer the return type of various builders instead of specifying them explicitly

* Further tweaking the generic buildQueryContext method and viz speicific buildQuery methodes per PR comments.

* git mv a renamed file.

* addressing additional pr comments

superset/assets/src/query/buildQueryObject.ts

@@ -0,0 +1,18 @@
+import FormData, { getGranularity } from './FormData';
+
+// TODO: fill out the rest of the query object
+export interface QueryObject {
+  granularity: string;
+  groupby?: string[];
+}
+
+// Build the common segments of all query objects (e.g. the granularity field derived from
+// either sql alchemy or druid). The segments specific to each viz type is constructed in the
+// buildQuery method for each viz type (see `wordcloud/buildQuery.ts` for an example).
+// Note the type of the formData argument passed in here is the type of the formData for a
+// specific viz, which is a subtype of the generic formData shared among all viz types.
+export default function buildQueryObject<T extends FormData>(formData: T): QueryObject {
+  return {
+    granularity: getGranularity(formData),
+  };
+}