feat: add recursiveDelete() to Firestore

[thebrianchen]

This PR adds recursive delete functionality to the top level Firestore class in a method that recursively deletes all documents and subcollections at and under the specified level. If any delete fails, the promise is rejected with an error message containing the number of failed deletes and the stack trace of the last failed delete. The provided reference is deleted regardless of whether all deletes succeeded.

Note that this is still a BetaApi and may change in the future.

Example:

// Recursively delete a document reference and log the references of failures.
BulkWriter bulkWriter = firestore.bulkWriter();
bulkWriter.addWriteErrorListener(
  (BulkWriterException error) -> {
      if (error.getFailedAttempts() < MAX_RETRY_ATTEMPTS) {
        return true;
      } else {
        System.out.println("Failed write at document: " + error.getDocumentReference());
        return false;
      }
    }
  );
firestore.recursiveDelete(documentReference, bulkWriter);