add DSSE rekor type to support any DSSE envelope

Adds a DSSE envelope type to rekor.  If the DSSE envelope's payload is
an in-toto statement the in-toto subjects will be used as indices for
the envelope's rekord.  If the envelope's payload is within the server's
configured attestation size the payload will be stored as an
attestation.

Signed-off-by: Mikhail Swift <mikhail@testifysec.com>

cmd/rekor-cli/app/root.go

@@ -28,6 +28,7 @@ import (
 
 	// these imports are to call the packages' init methods
 	_ "github.com/sigstore/rekor/pkg/types/alpine/v0.0.1"
+	_ "github.com/sigstore/rekor/pkg/types/dsse/v0.0.1"
 	_ "github.com/sigstore/rekor/pkg/types/hashedrekord/v0.0.1"
 	_ "github.com/sigstore/rekor/pkg/types/helm/v0.0.1"
 	_ "github.com/sigstore/rekor/pkg/types/intoto/v0.0.1"

cmd/rekor-server/app/serve.go

@@ -31,6 +31,8 @@ import (
 	"github.com/sigstore/rekor/pkg/log"
 	"github.com/sigstore/rekor/pkg/types/alpine"
 	alpine_v001 "github.com/sigstore/rekor/pkg/types/alpine/v0.0.1"
+	"github.com/sigstore/rekor/pkg/types/dsse"
+	dsse_v001 "github.com/sigstore/rekor/pkg/types/dsse/v0.0.1"
 	hashedrekord "github.com/sigstore/rekor/pkg/types/hashedrekord"
 	hashedrekord_v001 "github.com/sigstore/rekor/pkg/types/hashedrekord/v0.0.1"
 	"github.com/sigstore/rekor/pkg/types/helm"
@@ -92,6 +94,7 @@ var serveCmd = &cobra.Command{
 			helm.KIND:         helm_v001.APIVERSION,
 			tuf.KIND:          tuf_v001.APIVERSION,
 			hashedrekord.KIND: hashedrekord_v001.APIVERSION,
+			dsse.KIND:         dsse_v001.APIVERSION,
 		}
 
 		for k, v := range pluggableTypeMap {

openapi.yaml

@@ -395,6 +395,23 @@ definitions:
         - spec
       additionalProperties: false
 
+  dsse:
+    type: object
+    description: DSSE object
+    allOf:
+    - $ref: '#/definitions/ProposedEntry'
+    - properties:
+        apiVersion:
+          type: string
+          pattern: ^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$
+        spec:
+          type: object
+          $ref: 'pkg/types/dsse/dsse_schema.json'
+      required:
+        - apiVersion
+        - spec
+      additionalProperties: false
+
   rfc3161:
     type: object
     description: RFC3161 Timestamp