Keep track of original filename in RRD's `FileHeader`

[teh-cmc]

Since #7091, it is now possible to work with multiple RRD files concatenated into a single io stream, but there is no way to demux the stream back into its individual sources since we lost this information in the process.

Maybe it would be nice for the FileHeader to hold to this information at some point.

pub(crate) struct FileHeader {
    pub magic: [u8; 4],
    pub version: [u8; 4],
+   pub filename: String,
    pub options: EncodingOptions,
}