Converters for other job inputs to Jobspec

[vsoch]

When a new tool is introduced to an ecosystem, a user should not be required to rewrite everything. When possible, existing specifications for work (e.g., batch jobs) should convert over easily. Toward this goal (and using this strategy) I think we want to allow the following submission types for rainbow:

• simple: for basic users, a command and the most basic of parameters will be provided and converted to a Jobspec.
• jobspec: for advanced users, a Jobspec can be provided directly.
• Kubernetes job: for Kubernetes users, a batchv1/Job can be provided that will be converted to a Jobspec.

We will likely start with the first (simple) and then work on implementations for the latter. The converters will need to be implemented here, and will allow different communities to better harness Jobspec.

[vsoch]

And this should come with associated functions to generate them, and since they are versioned, we would want the Version field to be populated appropriately.