Allow closure segmenter util to generate a complete segmentation plan including table keyed segments

[garretrieger]

Adds three new config settings:

• generate_table_keyed_segments: when enabled a table keyed segment will be added to the output plan for each merge group (including the 'ungrouped' one).
• base_segmentation_plan: any segmentation plan settings here are copied to the output plan. Useful for configuring settings related to the table keyed encoding.
• MergeGroup::name: assigns a name to each merge group so they can be more easily identified in the log output.

This simplifies plan creation by avoiding needing a separate run of util/generate_table_keyed_config.