[SPA++] Add Model Contribution APIs in HstRequest and HstRequestContext

Add the following API:

HstRequest#setModel(String name, Object model);
<T> T HstRequest#getModel(String name);
Enumeration<String> HstRequest#getModelNames();
Map<String, Object> HstRequest#getModelsMap();

Also, HstRequest#getAttribute(name) should fall back to #getModel(name) when no request attribute is found. Otherwise, any access to the model in JSP/FreeMarker (which convert request attributes to the models in view automatically) can fail.
And, HstRequest#getAttributeNames() should combine model names for the same reason.

The same pattern applies to HstRequestContext.

By providing the APIs in both HstRequest and HstRequestContext, a component can decide where to contribute. The former one is supposed to be non-global ones whereas the latter one is supposed to be global one in the request processing.