Optional hardware attributes for Slurm agents

https://redmine.teklia.com/issues/5075

We should assume that Ponos agents running tasks on Slurm instead of Docker have infinite hardware requirements, as the real task assignation is managed by Slurm itself. For this reason, we need a way to determine whether an agent is using Slurm or Docker, and we need to make the agent hardware requirements optional.

A new Agent.mode field should be added to Ponos agents, as an EnumField using a new AgentMode enum, with two values: docker and slurm. The field defaults to docker.

The cpu_cores, cpu_frequency, and ram_total fields should be made nullable.

A new check constraint should verify that cpu_cores, cpu_frequency and ram_total are not null, but only when the mode is not slurm. That is doable with an OR: mode = 'slurm' OR (cpu_cores IS NOT NULL AND ...).