Add runtime handler support

[Random-Liu]

Fixes #382.

Example for containerd:

$ crictl runp sandbox.json
a7f2789dce1d2144331b2d9c42f5ccf771a2d5841076868bcd664cc351c95219
$ crictl inspectp a
...
    "runtime": {
      "runtimeType": "io.containerd.runtime.v1.linux",
      "runtimeEngine": "",
      "runtimeRoot": ""
    },
...

$ crictl runp --runtime=runsc sandbox.json 
c112976cb6caa43a967293e2c62a2e0d9d8191d5109afef230f403411147548c
$ crictl inspectp c
...
    "runtime": {
      "runtimeType": "io.containerd.runtime.v1.linux",
      "runtimeEngine": "/usr/local/sbin/runsc",
      "runtimeRoot": "/run/containerd/runsc"
    },
...

/cc @yujuhong @tallclair

[k8s-ci-robot]

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In response to this:

Fixes #382.

/cc @yujuhong @tallclair

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[yujuhong]

We can change variable name from context to ctx in this file, and just import the context package as it is.

[Random-Liu]

Done

[tallclair]

Awesome, thanks for this!

[feiskyer]

/lgtm