Add endpoint for external to get information about UDFs

Currently we have implemented some Pinot UDFs, however they are not transparently pushdown-able from external query engines like Presto.

The proposed change will add a new endpoint in PinotController. The endpoint will provide information of Pinot UDFs, so that we will have a generic way to understand what can be pushed down to pinot.

In general I’m thinking of the fields we need are:

`function_name`
`function_type`
`arguments_types`
`return_type`

e.g.

[
  {
    "function_name": "segmentPartitionedDistinctCount",
    "function_type": "AGGREGATION",
    "arguments_types": [ "ANY" ],
    "return_type": [ "LONG" ]
  },
  ...
]

[Jackie-Jiang]

How do you plan to maintain this endpoint? Not sure if this can help with query push down as the behavior must be identical on presto side and pinot side. IMO it is safer to manually select functions to push down instead of using name matching. It can cause unexpected behavior if the behavior is not exactly the same.

[yupeng9]

How do you plan to maintain this endpoint? Not sure if this can help with query push down as the behavior must be identical on presto side and pinot side. IMO it is safer to manually select functions to push down instead of using name matching. It can cause unexpected behavior if the behavior is not exactly the same.

We can use reflection to auto generate this information or we can get it from TransformFunctionFactory. I think this is useful when we have many releases of Pinot and Presto and we are keep adding new functions.

Also cc: @yupeng9

+1 This helps a lot minimizing the efforts of syncing the functions between Pinot and Presto. Moreover, I feel there are functions from Pinot that may not be able to introduce the counterpart in Presto. For example, the IdSet function introduced by https://github.com/apache/incubator-pinot/pull/5926