[release/1.7] CRI: Support Linux usernames for !linux platforms

[dcantah]

Backport: #8464

The oci.WithUser option was being applied in container_create_linux.go instead of the cross plat buildLinuxSpec method. There's been recent work to try and make every spec option that can be applied on any platform able to do so, and this falls under that. However, WithUser on linux platforms relies on the containers SnapshotKey being filled out, which means the spec option needs to be applied during container creation.

To make this a little more generic, I've created a new platformSpecOpts method that handles any spec opts that rely on runtime state (rootfs mounted for example) for some platforms, or just platform options that we still don't have workarounds for to be able to specify them for other platforms (apparmor, seccomp etc.) by internally calling the already existing containerSpecOpts method.

(cherry picked from commit 66307d0)

[samuelkarp]

Looks like this is just in sbserver. Does this need to be ported to the old CRI server package as well?

[dcantah]

@samuelkarp In the original change I'd only done it for the sandbox server as it uses the sandbox's Platform rpc to know whether to run some logic: https://github.com/containerd/containerd/pull/9015/files#diff-15877ae3602deacafa14a8688676328a93c85215593babe9f4b8a7bd46c2803bR483-R503. I could try and adapt for the regular CRI plugin but I don't think it's as useful there. One use case being this essentially adds support for any operating system that wants to virtualize linux to be able to get the linux username filled out, as if the sandbox is ultimately going to run a Linux container it will let us know via controller.Platform. All we've got is GOOS to key off of in the regular CRI plugin, so we don't have the hint

[dcantah]

/retest

[dcantah]

@samuelkarp ptal