Allow cli overrides of render params

## Summary

Allow command line overrides of render params

## Motivation

Overriding values is a common requirement when building reusable pipelines. Allowing values in an existing render.yml file to be overridden at the command line gives flexibility without having to rewrite the yaml file programmatically. [jsonnet supports both "external variables" and "top-level arguments" as command line arguments.](https://jsonnet.org/ref/language.html)

## Describe alternatives you've considered

1. Rewriting the render.yml file before running `shore render`

## Additional context

An example of the proposed syntax:

```bash
shore render --override foo=bar
```
where `foo` is a YAML key path, and `bar` is a YAML value.