[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/spec/javascripts/superset-ui/WordCloudBuildQuery.test.ts

@@ -0,0 +1,15 @@
+import buildQuery from 'src/visualizations/wordcloud/buildQuery';
+
+describe('WordCloud buildQuery', () => {
+  const formData = {
+    datasource: '5__table',
+    granularity_sqla: 'ds',
+    series: 'foo',
+  };
+
+  it('should build groupby with series in form data', () => {
+    const queryContext = buildQuery(formData);
+    const [ query ] = queryContext.queries;
+    expect(query.groupby).toEqual(['foo']);
+  });
+});