[Feat] Add batch function and a collection document delete logic, FirestoreMethod type

[Feautre]

• Implemented logic to delete all documents in a collection using batch.
• improved, including saving some redundant logic using Combine+extension.
• Previously, when receiving collection documents, the endpoints response DTO type was specified as a single type even if it had to be received as an array, but now it must be specified as an array. 🤩

[Example]

1. When deleting all documents in a collection, set the FirestoreMethod type to deleteADocument.
2. And then, call a request(endpoint:) func using FirestoreService object ! delete it.
3. Before deletion, FirestoreAccessible's documentRef must be nil and collectionRef must be specified as the deletion target path in the endpoint.

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References:

• Firebase Document link
• Firebase | WriteBatch document link