Base64 signature

[nenaddedic]

This is a fix for #427.

Changes:

• When verifying an attestation, treat the signature as base64-encoded.
• When creating an attestation, create it with a base64-encoded signature.

This change breaks backwards compatibility. Attestations verifiable with old code are no longer verifiable with the new code.

Users of GAP will have to recreate the attestations.

Effects on ISP will be limited: attestations previously created by ISP will become invalid, but ISP will create new valid ones for images passing the ISP. The only tricky case is that of a deployment previously passing the ISP (under the old code), but which is now not passing. In this case, new Pods will trigger ISP re-evaluation (because invalid attestations, under the new code), which will fail.

[aysylu]

A few minor comments about severity of logging and a nit on fn naming.

[aysylu]

@nenaddedic: before merging, could you please confirm integration tests pass locally for you? The kokoro results look like a flake, but I'd like to confirm first.

[nenaddedic]

@nenaddedic: before merging, could you please confirm integration tests pass locally for you? The kokoro results look like a flake, but I'd like to confirm first.

@aysylu yes, they pass locally