Expose NamedFileProtocolInfo's Header types

[ocadaruma]

Motivation

• To get file info, we have to pass buffer which has sufficient capacity
  • https://github.com/rust-osdev/uefi-rs/blob/master/src/proto/media/file/mod.rs#L115
• When the file-name is known, we can allocate the buffer of exactly required size
  • that is, size_of::<Header>() + name_length
• It'd be helpful to expose Header types so that we can calculate the buffer size based on that

[GabrielMajeri]

Thanks, looks good to me.