What is the purpose of this change?

This adds the --sparse option as well as the --sparse-min-size option in order to turn sequences of nulls larger than --sparse-min-size into sparse blocks by not writing to those locations in the destination.

For multithreaded copying, this is implemented for every backend that has the OpenWriterAt feature. For single threaded copying, a reference implementation has been done for backends/local.

Note that this a reopening of changes presented in #8619. I just felt it more appropriate to create a new PR as the changes here extend beyond linux filesystems that support FS_IOC_FIEMAP.

Was the change discussed in an issue or in the forum before?

https://forum.rclone.org/t/possible-to-use-rclone-with-sparse/45189

Checklist

• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• I'm done, this Pull Request is ready for review :-)