[TECH DEBT] Make `is_managed` a `What.IS_MANAGED`

[JCZuurmond]

Problem statement

Migrating managed tables is signaled differently in the table migrator, than the others:

ucx/src/databricks/labs/ucx/hive_metastore/table_migrate.py

Lines 147 to 148 in 2209174

	if src_table.src.is_managed:
	return self._migrate_managed_table(managed_table_external_storage, src_table)

Solution

Move this marker to the What