Extend ISourceLookUpProvider to be able to specify whether the source is Local (VSCode) or Remote(DAP Server)

[VladimirMakaev]

Context

I'm building a DAP server based on core package. I.e. without eclipse plugin. Currently it's impossible to set "sourceReference" when returning types in "stacktrace" request.

Having a distinction between Local and Remote sources allows an implementation of ISourceLookUpProvider to instruct the underlying request handlers whether a follow up with "source" request and have it served via getSourceContents

This is also ABI compatible with whoever could implement ISourceLookUpProvider already as current logic is identical to SourceType.Local

Test Plan

I've tested this works with my DAP server

[VladimirMakaev]

This should address #259 without breaking changes in current extension, allowing to have a custom implementation of ISourceLookUpProvider as a solution