Add NOTICE.txt and cgmanifest tool (dotnet#8843)

- Add generated NOTICE.txt with hand included OpentTK license (https://github.com/mono/opentk/blob/master/Documentation/License.txt) not found by CG
- Add small dotnet core tool that updates cgmanifest.json based on new submodules/mono
- Internal User Story: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1140262

Author: chamons