[v1.13] Add `usedDatabaseSize` to stats

[Strift]

Following the changes related to Meilisearch v1.13.0 mega issue:

Description

Update the stats object to include the new usedDatabaseSize field.

For further reference, see the related Meilisearch engine issue.

Specifications

• Modify the stats API to include the usedDatabaseSize field in SDK responses.