This is a new `DataSource` that can be used to read from a `FileDescriptor`.
Limitations:
- The provided file descriptor must be seekable via lseek.
- There's no way to duplicate a file descriptor with an independent position (it
would be necessary instead for the app to provide a new FD). Therefore this
implementation will only work if there's one open data source for a given file
descriptor at a time.
PiperOrigin-RevId: 649443584
adf1c7915d