Adding a CLI tool for this project

## 🌱 Describe your Feature Request

Could you add a CLI to coreml that expose some of the main features from the command line?

## How can this feature be used?

I would like to use a command such as:

```
coreml convert my_model.pth
```

## Describe alternatives you've considered

Creating a python script everytime to load the model and convert it to disk

## Additional context

It could also be loaded from the python3 command line such as:

```
python3 -m coremltools convert ...
```

Using [Click](https://click.palletsprojects.com/en/8.1.x/) could be helpful to build quickly a CLI tool

Start by reviewing the `python3 -m coremltools` entry point and the existing model-conversion interfaces. Define which main features the CLI should expose, including the proposed `coreml convert my_model.pth` flow, and verify that equivalent command-line and module invocation work as intended.

Written by the indexing model from the issue text.