Mark SandboxSnapshot init as public for sandbox plugins

[ArnavReddy]

Changed SandboxSnapshot initializer from package init() to public init() to enable external package consumers to create and use SandboxSnapshot instances in their custom Sandbox plugins.

The SandboxSnapshot is used for container list functionality

[dcantah]

CI's failing for an unrelated reason, but:

Test testRunCommandTmpfs() passed after 16.504 seconds.

All of the cli tests are taking 15+ seconds which seems insane.. Wonder if stop() regressed somehow