idea: Expose services options so that users can working on struct directly?

[Xuanwo]

Many users have code like the following:

pub enum StorageParams {
    Azblob(StorageAzblobConfig),
    Fs(StorageFsConfig),
    Ftp(StorageFtpConfig),
    Gcs(StorageGcsConfig),
    Hdfs(StorageHdfsConfig),
    Http(StorageHttpConfig),
    Ipfs(StorageIpfsConfig),
    Memory,
    Moka(StorageMokaConfig),
    Obs(StorageObsConfig),
    Oss(StorageOssConfig),
    S3(StorageS3Config),
    Redis(StorageRedisConfig),
    Webhdfs(StorageWebhdfsConfig),
    Cos(StorageCosConfig),

    /// None means this storage type is none.
    ///
    /// This type is mostly for cache which mean bypass the cache logic.
    None,
}

Maybe we can expose those config directly from opendal so that they don't need to repeat.