Add dotnet as submodule and update SDK build strategy

-Switched to using dotnet/dotnet as per Microsoft’s new build strategy
-Hence, fetching the coreesponding dotnet/sdk, dotnet/runtime , dotnet/msbuild ,dotnet/roslyn, dotnet/aspnetcore and dotnet/templating commits from src/manifest-json in the dotnet/dotnet repository, in order to build the SDK.
-Adding dotnet as a submodule to facilitate this process.

Signed-off-by: shreya <raoshreya412@gmail.com>

Author: ShreyaLaxminarayan

.gitmodules

@@ -19,3 +19,6 @@
 [submodule "templating"]
 	path = templating
 	url = https://github.com/dotnet/templating
+[submodule "dotnet"]
+	path = dotnet
+	url = https://github.com/dotnet/dotnet.git