Implement len(GroupBy) and ngroups for DataFrame API #20960
Description
Imported from Jira BEAM-13968. Original Jira may contain additional context.
Reported by: yeandy.
Subtask of issue #20958
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Imported from Jira BEAM-13968. Original Jira may contain additional context.
Reported by: yeandy.
Subtask of issue #20958