Allow any key/value pair under `mlops.model:`; support $variables$ for `mlops:`

[ReinhardKeil]

Problem

• Targets may require adjustments for the mlops: parameters. This can be solved by using $variables$ in the mlops: section.
• Some MLOps system may require additional information about the model. This can be solved by a flexible key: value list under mlops.model:. Note that clayer is really just a key/value pair - there is extra handling required for clayer.

Proposed changes

The mlops: node could then look like below:

  mlops:
    description: TinyCNN int8 image classifier for Ethos-U85
    npu:
      type: $NPU$
      macs: $Ethos-Macs$
    vela:
      system: $Ethos_system$  # Ethos_U85_SYS_DRAM_Mid # system-config from the Vela config
      memory: Shared_Sram # memory-mode from the Vela config
    model:
      clayer: $AI-Layer$
      key1: value1
      key2: $variable2$
    simulator:
      target: SSE-320-U85

In case that a variable is undefined, it should expand into an empty string (meaning that the mlops key has no value in the *.cbuild-mlops.yml file.