Accepts a map of setting-type to setting DTO. Semantics:

• Key absent from payload: setting is not touched.
• Key present with non-null value: upsert (create or update) the setting.
• Key present with explicit null value: delete the setting from the database.

All changes are applied atomically. If any setting fails validation
or applicability checks, the entire transaction rolls back and the
response contains per-setting error details.

Supported setting types:

• datetime-strategy: Controls how datetime values with timezone offsets
  are stored. Applicable only to DATETIME fields.
• collection-field: Marks a STRING scalar field as containing delimited
  collection data.