loki: generate dataplane-compliant logs dataframes (behind feature flag) (#69909)

* add feature flag

* prepare tests

* enable dataplane-format in loki-backend

pkg/tsdb/loki/testdata_dataplane/streams_parse_errors.json

@@ -0,0 +1,29 @@
+{
+  "status": "success",
+  "data": {
+    "resultType": "streams",
+    "result": [
+      {
+        "stream": { "__error__": "LogfmtParserErr", "place": "moon" },
+        "values": [
+          ["1655448591000000000", "\"hello1"],
+          ["1655448594000000000", "\"hello4"]
+        ]
+      },
+      {
+        "stream": {
+          "place": "moon",
+          "text": "hello3"
+        },
+        "values": [["1655448592000000000", "text=hello2"]]
+      },
+      {
+        "stream": {
+          "place": "moon",
+          "text": "hello4"
+        },
+        "values": [["1655448593000000000", "text=hello3"]]
+      }
+    ]
+  }
+}