Add support for several process directives

[bentsherman]

Close #27

This PR adds support for the following process directives:

• disk
• machineType
• resourceLabels
• time

While the CLI options can already be specified via float.commonExtra, the process directives are more user-friendly and can also be applied on a per-process level.

Also adds custom config options for --vmPolicy and --migratePolicy, since these options are some of the most valuable parts of Float but also have the most complicated syntax, we should try to make them as user-friendly as possible.

I also added a few default labels for things like process name, task name, session ID, etc. But feel free to remove them if you don't think they are needed. I copied them from the K8s executor.

I added unit tests for these options but haven't tested them end to end. Let me know if you need any help with testing.

Not urgent or groundbreaking but these settings would be nice to have for the initial blog post.

[jealous]

Thanks for the contribution @bentsherman.

[jealous]

For the custom tag names, some platforms such as GCP does not support upper case or characters like ., / or space. For now, I will try to replace them with -

[jealous]

I update the rootVolSize to imageVolSize because image volume is where we run the container where the actual work load happens. The root volume only contains the OS for VM.

[bentsherman]

Both fair points. I realized also that we probably need to replace : as well, at least while nf-float uses the CLI, since that character is used to separate the key/value.