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" ]
},
...
]
```

Start in PinotController and trace how existing controller endpoints expose metadata. Define the endpoint response around the listed function_name, function_type, arguments_types, and return_type fields, using the provided UDF example as a reference. Done means external query engines can retrieve Pinot UDF information in the proposed format.

Written by the indexing model from the issue text.