feat: add support for gVisor runtime

[acuteaura]

What this PR does / why we need it:

Handles the "configure containerd" part of setting up gVisor, since it can't be cleanly done in an OSP.

What type of PR is this?

/kind feature

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

Adds `--containerd-gvisor-runtime` flag, which, if set, adds a `runsc` runtime with the given runtime type (typically `io.containerd.runsc.v1`) to the containerd config.

Documentation:

https://gvisor.dev/docs/user_guide/containerd/quick_start/

[kubermatic-bot]

@acuteaura: Adding the "do-not-merge/docs-needed" label because no documentation block was detected, please follow our documentation process to remove it.

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[kubermatic-bot]

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[kubermatic-bot]

Hi @acuteaura. Thanks for your PR.

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