Add `process.group` fields to generated schemas

[mjwolf]

Add process.group fields as the effective group for the process schema. process.group was (probably mistakenly) being excluded from the schema, as it wasn't part of schemas/subsets/main.yml. The other related process group fields are already included in the schema (real_group, saved_group), and process.group is defined with field re-use in group.yml.

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