bake: composable attributes for attestations support

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

__tests__/.fixtures/bake-03-default.json

@@ -3,6 +3,16 @@
     "default": {
       "context": ".",
       "dockerfile": "Dockerfile",
+      "attest": [
+        {
+          "mode": "max",
+          "type": "provenance"
+        },
+        {
+          "disabled": "true",
+          "type": "sbom"
+        }
+      ],
       "cache-from": [
         {
           "scope": "build",

__tests__/.fixtures/bake-03.hcl

@@ -13,6 +13,10 @@
 // limitations under the License.
 
 target "default" {
+  attest = [
+    "type=provenance,mode=max",
+    "type=sbom,disabled=true",
+  ]
   cache-from = [
     "type=gha,scope=build",
     "user/repo:cache",