Add a script to analyze Application Settings

What does this MR do and why?

This MR adds a script that does several thing:

• Extract ApplicationSetting attributes from structure.sql and create individual YAML files under db/docs/application_settings/
• Cross-check the API documentation at doc/api/settings.md for consistency and extract description from it (this is mostly for ease of understanding the purpose of settings, and also to detect attributes that aren't exposed/documented in the API documentation)
• Generate statistics and a table for ApplicationSetting attributes based on the data from db/docs/application_settings/ files

The goal of individual YAML files under db/docs/application_settings/ is to populate the clusterwide attribute manually so that each attribute is classified as cell-local or clusterwide.

This is needed for the Phase 7 of Cells 1.0: &13165

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