Duplication of fields across *_schema.yaml files

[yifanmai]

Many fields are duplicated across *_schema.yaml files. Here is how to deal with some of them.

• adapter: This should be auto-inferred from the docstrings of AdapterSpec. Done in Populate schema adapter fields from AdapterSpec docstrings #2611.
• metrics: This could be moved to a metric_metadata.yaml file that is parallel to model_metadata.yaml. Or it could be inferred from a metadata() method on metrics objects. Note: It may not be possible to deduplicate this because there can be different Metric objects that use the same metric names - we don't enforce that the names are unique in a global namespace.
• perturbations: This should be moved to a perturbation_metadata.yaml file that is parallel to model_metadata.yaml. Doesn't seem to be a big problem because perturbations are rarely used.
• metric_groups: stay in schema.yaml.
• run_groups: There is an argument that "leaf" run groups i.e. scenarios should go in some scenario_metadata.yaml file. But it is possible that users will want to change which metrics groups are used for scenarios, so it should stay in schema.yaml.