Allow grouping and providing extra metadata for datasets, methods and metrics

[rcannood]

Example:

__merge__: /src/api/comp_metric.yaml
name: clustering_overlap
info:
  metric_type: graph
  metrics:
    - name: ari
      label: ARI
      summary: ...
      description: ...
      references:
        doi:
          - 10.1038/s41592-021-01336-8
          - 10.1007/bf01908075
      links:
        homepage: https://scib.readthedocs.io/en/latest/
        documentation: https://scib.readthedocs.io/en/latest/api/scib.metrics.silhouette_batch.html
        repository: https://github.com/theislab/scib
      min: 0
      max: 1
      maximize: true
      group: mygroup
      metadata: # always a <string, atomic value> pair
        icon: https://path/to/foo