Add fct_event_namespace_monthly to Data Catalog

Why is this change being made?

In order to truly function as handbook-first, we need to keep the Data Catalog in sync with changes to the EDM. This MR adds a new fact, common.fct_event_namespace_monthly, to the dbt Data Dictionary for product data models.

This will be merged at the same time as the Data MR so that the Data Catalog is not out of sync with Snowflake

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