Add workloadattestor v2

[arndt-s]

Adds v2 of workloadattestor plugin interface that allows to attest arbitrary references. See Broker API in spiffe and related SPIRE PR (referenced below)

SPIFFE PR: spiffe/spiffe#340
SPIRE PR: spiffe/spire#6594

[arndt-s]

This needs some more description.

[matheuscscp]

@amartinezfayo @MarcosDY @sorindumitru Yesterday one of you asked me if introducing this v2 for workloadattestor was really necessary, my conclusion today is: yes, unless the SPIFFE/SPIRE maintainers are ok with an abstraction in the SPIRE implementation that does not correctly express the WorkloadReference standard in the SPIFFE Broker API.

[sorindumitru]

@amartinezfayo @MarcosDY @sorindumitru Yesterday one of you asked me if introducing this v2 for workloadattestor was really necessary, my conclusion today is: yes, unless the SPIFFE/SPIRE maintainers are ok with an abstraction in the SPIRE implementation that does not correctly express the WorkloadReference standard in the SPIFFE Broker API.

I still think this should be possible. We recommend using UnimplementedWorkloadAttestorServer in our workloadattestor plugins, so if we add a new method to the V1 interface it should work in most places without recompiling. It wouldn't work for the built-in plugins because we use UnsafeWorkloadAttestorServer, but that's ok.

What I didn't check, but it should also work, is loading a plugin compiled against an old version of the interface. I think it should also return Unimplemented in that case.

In the SPIRE code we can then use the Unimplemented code to fallback to attempting to attest using a PID reference, if that's what was provided to us. And similar for the Attest call, if it isn't implemented we fallback to AttestReference with a pid reference.

[matheuscscp]

Superseded by: #74