To break the cyclic dependency of cri plugin and podsandbox plugin,
we define a new plugin type of SandboxesServicePlugin and when cri init
it's own client, it will add the all the controllers by get them from
the SandboxesServicePlugin.
when podsandbox controller init it's client, it will not Require the
SandboxesServicePlugin.
Signed-off-by: Abel Feng <fshb1988@gmail.com>
make containerd extensible to support more sandbox controllers
registered into containerd by config.
we change the default sandbox controller plugin's name from "local" to "shim".
to make sure we can get the controller by the plugin name it registered into
containerd.
Signed-off-by: Abel Feng <fshb1988@gmail.com>
