Implement or skip message_header.dev_fields

[arosenb2]

When reading a file with dev_fields, the current implementation ends up with panic.

See https://github.com/zzyandzzy/fit-rust/blob/main/src/lib.rs#L44-L46

if message_header.dev_fields {
    unimplemented!("message_header.dev_fields is unimplemented");
}

This doesn't seem like it should be a blocker for processing the rest of the file, so I suggest to either incorporate it or log and skip it.