[Feature Request] Allow the index engine to use pre-calculated index files for popular jars

### Background

Some complied class files do not contain parameter name information for the methods (for example, a lot of classes in JDK 1.8 do not have such information). Thus, in JDT, the index files for this kind of jars do not have the parameter names either. The result of that is: when user trigger the completion for the first time, JDT needs to take a lot of time 'warm up' the `SourceMapper` cache, and then get the parameter names to show the completion list.

### Feature Request

Allow the index manager use pre-calculated index file for some popular jars, like jars in JDK 1.8.

### Benefits

Below are the benefits that I think we can get if we support this feature:

1. The pre-calculated index file can leverage the sources and contains parameter name information, thus speed up the completion performance.
2. Save time to build the index files if we already have a pre-calculated index file somewhere.
3. We can even let the Index manager use the same pre-calculated index files if different workspace uses the same jars. For example, two workspaces all use JDK 1.8 in their projects. If we have a shared index for such case, it can save considerable disk space considering that the index file for a JDK usually takes 30-40MB.