`GenerateChecksumItems` task should have an option for generating Linux-format checksum files

`GenerateChecksumItems` generates contents like this:

`myfile.tar.gz.sha512`:
```
B41DB3C12FA66C20AFB6DBD57B2468F0106010091D01393071B3263095EC5CD58CDEC58A87C67554DACBFDE20C9AD3A34D9CDC22A4CE0A126EBBA6E19BB03EF6
```

However, linux tools like `sha512sum` expect files in a format like this:

```
b41db3c12fa66c20afb6dbd57b2468f0106010091d01393071b3263095ec5cd58cdec58a87c67554dacbfde20c9ad3a34d9cdc22a4ce0a126ebba6e19bb03ef6 myfile.tar.gz
```

Lowercase checksum, followed by two spaces, followed by the file name, ending with a single LF newline (not CRLF!). See [sha512sum manpage](https://man.archlinux.org/man/sha512sum.1) for more details.

`GenerateChecksumItems` task should have an option to generate files in this format.