[Request] Annotate/comment to indicate added transitive dependencies.

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Feature description

As suggested by a user, new NuGet dependencies that are added to a `.csproj`, etc. could be annotated with a comment to indicate that the transitive dependency was added to allow the package upgrade and that at some point in the future it is a candidate to be manually removed.

Start by locating the NuGet and .csproj update-generation code in dependabot-core, then inspect how newly added transitive dependencies are represented. Done means added transitive dependencies are annotated with a comment explaining why they were added and that they may be manually removed later.

Written by the indexing model from the issue text.