Add a registry infrastructure for SerializationFormat implementations,
enabling registration and instantiation of different serialization
formats.
For example:
```c++
static SerializationFormatRegistry::Add<MyFormat>
RegisterFormat("MyFormat", "Description");
```
Formats can then be instantiated by name using `makeFormat()`.
The patch also updates the SerializationFormat base class to accept
FileSystem for virtualising reading inputs eg. by using file overlays in
the future.
Assisted-by: claude
rdar://169192127
