Fix JSON parser handling for certain objects (#7162)

This PR also adds some comments and simplifies
the code. Primary handling is done to ensure
that we make sure to honor cached buffer.

Added unit tests as well

Fixes #7141

pkg/s3select/json/data/10.json

@@ -0,0 +1,12 @@
+[
+ {
+	 "key_1": "value",
+	 "key_2": "value"
+ }
+]
+[
+	{
+		"key_1": "value2",
+		"key_2": "value3"
+	}
+]