Missing projects and their references

[RaeesAbbas]

This repo has two missing project references i.e

<ProjectReference Include="..\..\..\LTI\LtiAdvantage\src\LtiAdvantage.IdentityModel\LtiAdvantage.IdentityModel.csproj" />
   <ProjectReference Include="..\..\..\LTI\LtiAdvantage\src\LtiAdvantage\LtiAdvantage.csproj" />

Please add missing projects and references so that we can move further for a sample idea of LTI Advantage Library.

[willwm]

@RaeesAbbas - This PR should (hopefully) address your issue: https://github.com/LtiLibrary/LtiAdvantageTool/pull/8/files

(Changes include the addition of LtiAdvantage as a submodule.)