[CLI] Generate only training assets in existing application

When using Model Builder for training, the following files are created as a result of the training process in the project from which "Add > Machine Learning Model" is selected:

- *.mbconfig
- *.training.cs
- *.consumption.cs
- *.zip (Model file)

Optionally, in the Consume step, users have the option to generate a new project from the Console and Web API templates provided. The same files are copied over to the new project and the user can use them to consume their model.

In the CLI however, a new console application with the mbconfig, training, consumption, and zip file inside it is created automatically.

**Proposal**:
- Only generate the *.mbconfig, *.training.cs, *.consumption.cs, and *.zip files inside the project where the `mlnet` command is run from.
- Make the generation of a new .NET project for model consumption optional.

Doing so will make the workflows feel similar between Model Builder and the CLI. Additionally, it'll simplify the process of using the model in an existing app because there's no need to go through the extra step of adding a reference to the newly generated project.