Feature Request: Magic Numbers at end of File/Struct

Nobody has claimed this yet.

Hi, some formats, eg Parquet have a Magic Number at the end of the file, also others have them at the end of headers eg Zstd Skippable Frames.

Although it is possible to define a struct like this

```
...
#[brw(magic = 0x8F92EAB1_u32)]
pub dummy: (),
}
```
however, it feels like a bit clunky.

Would it be possible to either support the following
```
...
#[brw(magic = 0x8F92EAB1_u32)]
}
```

or (maybe better for documentation purposes)
```
#[brw(end_magic = 0x184D2A5E_u32, little)]
pub struct MyStruct {
```

This would then allow a format to have start and end magic numbers.
```
#[brw(magic = 0x184D2A5E_u32,end_magic = 0x184D2A5E_u32, little)]
pub struct MyStruct {
```

Thanks for the excellent crate.

No files or tests are named. Start by tracing how the existing `magic` attribute is parsed and applied, then determine the intended syntax and behavior for validating a magic number at the end of a struct while preserving support for start and end markers. Done means the chosen API is implemented with coverage for the requested binary-format cases.

Written by the indexing model from the issue text.