Add ability to attach missing sources

[fbricon]

If a class file has no known source, a disassembled source is displayed instead.
There should be a way to easily attach missing sources to the jar containing the file in question.

Originally suggested by @jtsiros on https://marketplace.visualstudio.com/items?itemName=redhat.java#review-details

[fbricon]

@yaohaizh @testforstephen I think this could be implemented via a menu/command with stock vscode-java

[akaroml]

Propose a workflow:

1. Show "Open Folder" dialog to let users pick the location of the additional source
2. Persist the location into user settings - "java.additionalSourceLocations: []"
3. Language server follows the setting and works with those additional source locations

[akaroml]

@testforstephen can you take a look?

[testforstephen]

The ux is adding a context menu Update Source Attachment on the disassembled source editor (JdtContentProvider). When the user clicks the menu, pop up a Open File dialog to select the source jar/zip file. Then binding the source attachment to the binary path, and re-rendering the class file contents in VS Code.

[fbricon]

Fixed with #699, thanks to @testforstephen

[mikedoy]

When I attach source, where is this information stored? I'm not seeing it in my settings.json.