bug: signatures in DSSE envelopes are base64 encoded twice

[jiparis]

Since DSSE envelopes already encode them, and Chainloop uses Sigstore bundles, which use a protobuf version of DSSE struct, and since protobuf uses base64 to encode []byte fields, they are stored encoded twice.
I've observed that I needed to decode them twice during my TSA tests, and have also noticed that sigstore-go Bundle abstraction has helper methods to get the raw decoded value.

We need to make the change in such a way that it's backwards compatible with wrong signagures.