Add vTPM specification

[everzakov]

Add the vTPM (virtual Trusted Platform Module) specification to the documentation, config.go and schema description. Runtime uses this specification to create vTPMs and pass them to the container. This virtual module can be used to create quotes, signatures and perform Direct Anonymous Attestation.

Also, users can specify that vTPM should be manufactured with precreated certs / activated PCR banks and populated Endorsement Key Pair.

The following is an example of a vTPM description that is found under the path /linux/resources/vtpms:

    "vtpms": [
        {
            "statePath": "/var/lib/runc/myvtpm1",
            "statePathIsManaged": false,
            "vtpmVersion": "2",
            "createCerts": false,
            "runAs": "tss",
            "pcrBanks": "sha1,sha512",
            "encryptionPassword": "mysecret",
            "vtpmName": "tpm0",
            "vtpmMajor": 100,
            "vtpmMinor": 1
        }
    ]

This PR is based on #920

[tianon]

If I understand correctly, the idea is that a runtime is expected to start an instance of swtpm behind the scenes, and wire up the result inside the container. Is that accurate?

This is perhaps mirroring some of the concerns expressed in #920, but what's the benefit of doing that over running swtpm explicitly and mapping the device or socket from it?

To maybe help explain why this makes me nervous, what do we do if the container dies? The runtime is typically long gone at that point, so what makes sure swtpm shuts down? What if swtpm has a problem and shuts down before the container does? At the level of the runtime, there's no "orchestrator" monitoring processes, there's just a container process and a bunch of kernel resources tied to that process (most of which clean themselves up pretty reasonably when the container exits or dies).

Another aspect is how non-container runtimes (VMs, etc) are expected to implement this. If they can't support this, they should probably simply error, right? The same if swtpm is not installed?

So in short, why is the runtime layer the appropriate place for this and not, say, the orchestrators like containerd, Docker, kubernetes, etc?