File store to support context cancel

[Wwwsylvia]

Adding a directory to file.Store could take a long time and sometimes users may want to cancel it.
Currently the ctx parameter is not used in the Add method and context cancel is not supported. We should consider adding this support for better user experience.

oras-go/content/file/file.go

Lines 384 to 385 in 47d028a

	// Add adds a file into the file store.
	func (s *Store) Add(_ context.Context, name, mediaType, path string) (ocispec.Descriptor, error) {