solver: add attestations capability

This will eventually clients to query the server to confirm if
attestations are supported. However, for now, we can keep it disabled
until the gateway api stabilizes.

Signed-off-by: Justin Chadwell <me@jedevc.com>

solver/pb/caps.go

@@ -79,7 +79,8 @@ const (
 	CapMergeOp apicaps.CapID = "mergeop"
 	CapDiffOp  apicaps.CapID = "diffop"
 
-	CapAnnotations apicaps.CapID = "exporter.image.annotations"
+	CapAnnotations  apicaps.CapID = "exporter.image.annotations"
+	CapAttestations apicaps.CapID = "exporter.image.attestations"
 )
 
 func init() {
@@ -440,4 +441,11 @@ func init() {
 		Enabled: true,
 		Status:  apicaps.CapStatusExperimental,
 	})
+
+	// FIXME: enable once attestations gateway api is stable
+	// Caps.Init(apicaps.Cap{
+	// 	ID:      CapAttestations,
+	// 	Enabled: true,
+	// 	Status:  apicaps.CapStatusExperimental,
+	// })
 }