refactor: fix blob index assumption

[LHerskind]

Currently there is an assumption around the indexes of blobs when they are submitted.

public async sendBlobsToBlobSink(blockHash: string, blobs: Blob[]): Promise<boolean> {
    // TODO(md): for now we are assuming the indexes of the blobs will be 0, 1, 2
    // When in reality they will not, but for testing purposes this is fine
    if (!this.blobSinkUrl) {
      this.log.verbose('No blob sink url configured');
      return false;
    }