Make use of SFTP API for incus file operations

[stgraber]

Currently all the incus file operations are done through the individual REST API endpoints, which Incus then internally converts to SFTP interactions. This isn't exactly the most flexible or efficient.

It'd be good to change all the incus file operations to instead use GetInstanceFileSFTP to fetch a Go SFTP client and then perform the relevant filesystem operations through it.

[HassanAlsamahi]

Can I work on this? If that is okay, should the change be done in client/incus_instances and replace the REST API part of the functions CreateInstanceFile..etc with the SFTP part? or directly in the cmd/incus/files? I believe the first but wanted to verify.

[stgraber]

Assigned it to you!

The change should be done directly in cmd/incus/file.go, the Go client package should be a mostly clear mapping to the API so we don't want it to get too smart on that front.