refactor: rsync and scp into copytool pkg

[unsuman]

This PR refactors the file copying logic (rsync and scp) out of cmd/limactl and into a new package copytool.

Changes were made by creating a pkg/copytool with a common CopyTool interface, moving the rsync and scp command generation logic into this new package, and updating limactl copy and limactl shell to utilise the new package, which resulted in the removal of duplicated logic in shell.go.

Fixes

Yes, I think we should make a separate package rsync same as https://github.com/AkihiroSuda/alcless/blob/master/pkg/rsync/rsync.go
but this can be done by me in another PR?

Originally posted by @unsuman in #4429 (comment)

[unsuman]

Left with the refactoring of rsync in the cmd/limactl/shell.go file with these current changes.
Since there exists bats testing for the copy and shell --sync command, is there a need to add unit tests for this new copytool package?

[AkihiroSuda]

Needs rebase

[unsuman]

I'll rebase it once #4563 is merged