Allow chunked blob push

[shizhMSFT]

The current implementation of content/oci and registry/remote only supports monolithic blob upload where the descriptor must be known in advance.

If we want to support chunked blob push, we should have the implementations to implement a new interface like

type UnsafePusher interface {
	// UnsafePush pushes the content, matching the specified field in the expected descriptor.
	// - The `Digest` of the expected descriptor is recalculated if empty
	// - The `Size` of the expected descriptor is recalculated if less than zero
	// The recalculated descriptor is returned upon a successful push.
	UnsafePush(ctx context.Context, expected ocispec.Descriptor, content io.Reader) (ocispec.Descriptor, error)
}

This issue is to track the discussion on whether we should support chunked blob push with security considerations.

• Since the digest might be missing, the integrity of the content might not be guaranteed. That is, the pushed content might be different from expected.
• Since the size might be missing, arbitrary length or even infinite length of content might be written to the target without notice.

[sparr]

For possibly useful reference, containerd's content package implements a Writer that accepts multiple writes followed by a single Commit.

As someone currently implementing a wrapper interface to allow seamless choice between containerd's content package and oras-go's oci package, having similar APIs would be quite welcome.