ast: Adding rego.metadata.* built-in functions (#4537)

New functions:
* rego.metadata.chain(): returns the chain of metadata, starting from the active rule, going outward
* rego.metadata.rule(): returns the metadata for the active rule

Signed-off-by: Johan Fylling <johan.dev@fylling.se>

capabilities.json

@@ -2856,6 +2856,27 @@
         "type": "function"
       }
     },
+    {
+      "name": "rego.metadata.chain",
+      "decl": {
+        "result": {
+          "dynamic": {
+            "type": "any"
+          },
+          "type": "array"
+        },
+        "type": "function"
+      }
+    },
+    {
+      "name": "rego.metadata.rule",
+      "decl": {
+        "result": {
+          "type": "any"
+        },
+        "type": "function"
+      }
+    },
     {
       "name": "rego.parse_module",
       "decl": {