[SourceKit] Add mechanism to load plugins for request handling into SourceKit

This allows us to load plugins into the sourcekitd service to handle requests (currently only used for code completion). This allows us to implement parts of sourcekitd in Swift and outside of the compiler repository, making it easier to iterated on them because the compiler doesn’t need to be rebuilt.

unittests/SourceKit/SwiftLang/CloseTest.cpp

@@ -83,6 +83,7 @@ public:
     Ctx = std::make_shared<SourceKit::Context>(
         getSwiftExecutablePath(), getRuntimeLibPath(),
         /*diagnosticDocumentationPath*/ "", SourceKit::createSwiftLangSupport,
+        [](SourceKit::Context &Ctx){ return nullptr; },
         /*dispatchOnMain=*/false);
   }