[v1.26] Add tasks custom metadata

[Strift]

Description

Meilisearch v1.26 allows attaching custom metadata to document addition or update tasks.

We want to update this SDK's methods for documents addition and update to accept an optional metadata string.

POST /indexes/{indexUid}/documents
PUT /indexes/{indexUid}/documents
DELETE /indexes/{indexUid}/documents/{documentId}
POST /indexes/{indexUid}/documents/delete-batch
POST /indexes/{indexUid}/documents/delete
POST /indexes/{indexUid}/documents/edit
DELETE /indexes/{indexUid}/documents

For more information, read the usage guide.

Tasks

• [ ] Update all methods that interact with the listed API endpoints
• [ ] Add test cases