[v1.15] Add `batchStrategy` field to batches

[Strift]

This issue follows the changes related to Meilisearch v1.15.0 mega issue.

Description

Update responses from the batches API to contain the batchStrategy field.

For more context, see the related issue in Meilisearch: meilisearch/meilisearch#5511

Note: the Batches API was introduced in Meilisearch 1.12, it first needs to be implemented via #628

Tasks

• Update the batches-related methods to return the new field
• Add new test cases