[SPARK-12321][SQL] JSON format for TreeNode (use reflection)

An alternative solution for https://github.com/apache/spark/pull/10295 , instead of implementing json format for all logical/physical plans and expressions, use reflection to implement it in `TreeNode`.

Here I use pre-order traversal to flattern a plan tree to a plan list, and add an extra field `num-children` to each plan node, so that we can reconstruct the tree from the list.

example json:

logical plan tree:
```
[ {
  "class" : "org.apache.spark.sql.catalyst.plans.logical.Sort",
  "num-children" : 1,
  "order" : [ [ {
    "class" : "org.apache.spark.sql.catalyst.expressions.SortOrder",
    "num-children" : 1,
    "child" : 0,
    "direction" : "Ascending"
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.AttributeReference",
    "num-children" : 0,
    "name" : "i",
    "dataType" : "integer",
    "nullable" : true,
    "metadata" : { },
    "exprId" : {
      "id" : 10,
      "jvmId" : "cd1313c7-3f66-4ed7-a320-7d91e4633ac6"
    },
    "qualifiers" : [ ]
  } ] ],
  "global" : false,
  "child" : 0
}, {
  "class" : "org.apache.spark.sql.catalyst.plans.logical.Project",
  "num-children" : 1,
  "projectList" : [ [ {
    "class" : "org.apache.spark.sql.catalyst.expressions.Alias",
    "num-children" : 1,
    "child" : 0,
    "name" : "i",
    "exprId" : {
      "id" : 10,
      "jvmId" : "cd1313c7-3f66-4ed7-a320-7d91e4633ac6"
    },
    "qualifiers" : [ ]
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.Add",
    "num-children" : 2,
    "left" : 0,
    "right" : 1
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.AttributeReference",
    "num-children" : 0,
    "name" : "a",
    "dataType" : "integer",
    "nullable" : true,
    "metadata" : { },
    "exprId" : {
      "id" : 0,
      "jvmId" : "cd1313c7-3f66-4ed7-a320-7d91e4633ac6"
    },
    "qualifiers" : [ ]
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.Literal",
    "num-children" : 0,
    "value" : "1",
    "dataType" : "integer"
  } ], [ {
    "class" : "org.apache.spark.sql.catalyst.expressions.Alias",
    "num-children" : 1,
    "child" : 0,
    "name" : "j",
    "exprId" : {
      "id" : 11,
      "jvmId" : "cd1313c7-3f66-4ed7-a320-7d91e4633ac6"
    },
    "qualifiers" : [ ]
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.Multiply",
    "num-children" : 2,
    "left" : 0,
    "right" : 1
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.AttributeReference",
    "num-children" : 0,
    "name" : "a",
    "dataType" : "integer",
    "nullable" : true,
    "metadata" : { },
    "exprId" : {
      "id" : 0,
      "jvmId" : "cd1313c7-3f66-4ed7-a320-7d91e4633ac6"
    },
    "qualifiers" : [ ]
  }, {
    "class" : "org.apache.spark.sql.catalyst.expressions.Literal",
    "num-children" : 0,
    "value" : "2",
    "dataType" : "integer"
  } ] ],
  "child" : 0
}, {
  "class" : "org.apache.spark.sql.catalyst.plans.logical.LocalRelation",
  "num-children" : 0,
  "output" : [ [ {
    "class" : "org.apache.spark.sql.catalyst.expressions.AttributeReference",
    "num-children" : 0,
    "name" : "a",
    "dataType" : "integer",
    "nullable" : true,
    "metadata" : { },
    "exprId" : {
      "id" : 0,
      "jvmId" : "cd1313c7-3f66-4ed7-a320-7d91e4633ac6"
    },
    "qualifiers" : [ ]
  } ] ],
  "data" : [ ]
} ]
```

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10311 from cloud-fan/toJson-reflection.