feat(file-storage): add methods to check whether bucket exists

[getlarge]

Method for GC Storage:

// ...
  async bucketExists(): Promise<boolean> {
    const { storage, bucket } = this.config;
    try {
      const [exists] = await storage.bucket(bucket).exists();
      return exists;
    } catch (e) {
      return false;
    }
  }
  //... 

• Method for AWS S3:

  async bucketExists(): Promise<boolean> {
    const { s3, bucket: Bucket } = this.config;
    try {
      await s3.headBucket({ Bucket });
      return true;
    } catch (e) {
      return false;
    }
  }