Command to set default label

[kylewillmon]

The default label for a project determines which job is considered current for a project. This has impacts in search, monitoring notifications, and the policy result shown in the project list. Since it is so important, we need a CLI command to set the default label for a project

[kylewillmon]

I think this should go under phylum project update... But it is a bit of a bummer that the default label is set with the project preferences endpoint and not the project update endpoint... So that command would need to make 2 API calls now if we include default label there.