Use a dumb signed pod for padding #246
Description
With the plonky2 signature we had a verifier with a selector to disable the verification, but in the MainPod circuit we're always setting the selector to true and using a padding signature if necessary.
Nevertheless the padding signedpod is taken from an input signedpod, which means we can't make a MainPod that verifies signed pods with no input signed pods.
We should resolve this by having a default dumb signed pod that is built with no input key-values and a "dumb" private key. And then use this "constant" signed pod for padding when necessary.
Blocked by #236