refactor!: Combine GPU and simple Scaler into single Scaler

[yorik]

What ❔

Combine GPU and simple Scaler into single Scaler
Move code which interacts with other part into manager.rs.
Created Key trait and GpuKey and NoKey implementations.
Changed Scaler config.

Why ❔

To reduce copy of the almost the same code and number of bugs.

Is this a breaking change?

• Yes
• No

Operational changes

Requires following config change:

• Add into scaler_targets prover config like this:

  scaler_targets:
    - queue_report_field: prover_jobs
      deployment: circuit-prover-gpu
      min_replicas: 0
      max_replicas:
        cluster-foo:
          L4: 100
          T4: 200
        cluster-bar:
          L4: 100
          T4: 200
      speed:
        L4: 1500
        T4: 700

• Remove min_provers, max_provers, prover_speed.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zkstack dev fmt and zkstack dev lint.

ref ZKD-2229