Stream optional interfaces

[ahouene]

Per #3, and inspired by fs.FS optional interfaces (e.g. fs.GlobFS), this PR introduces two optional interfaces: WriterStorage and ReaderStorage.

// A ReaderStorage is an Interface providing an optimized way to create an io.ReadCloser.
type ReaderStorage interface {
	Interface
	// NewReader returns an io.ReadCloser, allowing stream reading
	// of named value from the underlying backend.
	NewReader(context.Context, string) (io.ReadCloser, error)
}

// A WriterStorage is an Interface providing an optimized way to create an io.WriteCloser.
type WriterStorage interface {
	Interface
	// NewWriter returns an io.WriteCloser, allowing stream writing
	// to named key in the underlying backend.
	NewWriter(context.Context, string) (io.WriteCloser, error)
}

Those interfaces are created using global functions:

func NewReader(ctx context.Context, st Interface, name string) (io.ReadCloser, error)

func NewWriter(ctx context.Context, st Interface, name string) (io.WriteCloser, error)

In case st doesn't satisfy ReaderStorage or WriterStorage, it will be wrapped in a struct, allowing abstraction but no performance gain.

Corresponding tests have been implemented.

Closes #3

[wojas]

Mostly nits and improvements, the logic LGTM, except for the fs writer.

[wojas]

First pass, will do a more in depth one soon.

[johanlantz]

This looks solid, I guess you have looked at other similar solutions like this one:
https://github.com/google/renameio

just to see we are not missing any corner cases?

[ahouene]

Thanks for the recommendation. I gave it a look and while it looks like it can handle a broader set of situations (that we don't need here), I didn't see something that would benefit us here.

[johanlantz]

Nothing to add since last time. If @wojas comments are all closed it seems like it's time to finalize this.