Expose config struct always

[tisonkun]

Now all the backend services has an associated Config type.

In my project I'd prefer to extra all the config into a separated crate (protos) and keep it as thin as enough, i.e., I need:

• Refer to configs said S3Config;
• Without refer to the whole S3 backend.

The config will later pass to the service crate and really initialize the backend (within the operator). And in this way in the protos crate I can depend on opendal = { version = ".." } without any features.

@Xuanwo what do you think?

[Xuanwo]

a separated crate (protos)

A separate crate might not work since we need to implement Configurator, which is highly coupled to its Builder. Maybe we can expose it in a new module like opendal::services::S3Config by placing it in services/s3_config.rs.