local invoke --env-vars should process .yml files as well - not just .json

### enhancement - feature request

`sam local invoke --env-vars` should also be able to process .yml files.
As of now, it has to be a .json file.

### Proposal

i.e. cloudformation does allow to load environment variables from a .yml file for example with
```yaml
provider:
environment: ${file(./env.yml)}
```

So it would be handy if I could reuse the file.

Thank you