Fix intoto index keys (#889)

* Fix intoto index keys

Ensure we include all appropriate index keys including:
- entire DSSE envelope SHA256 digest
- envelope (base64 decoded) SHA256 digest
- entire X509 signing certificate
- any relevant keys extracted from X509 signing certificate

Fixes: #872

Signed-off-by: Bob Callaway <bcallaway@google.com>

* consistently decode payload

Signed-off-by: Bob Callaway <bcallaway@google.com>

* rework error flows to be non-fatal

Signed-off-by: Bob Callaway <bcallaway@google.com>

* use canonicalized key, consistent sprintf

Signed-off-by: Bob Callaway <bcallaway@google.com>