sql: introduce FINALIZE for custom aggregate

This patch introduces a way to create a FINALIZER for user-defined
aggregate functions.

Closes #2579

@TarantoolBot document
Title: User-defined aggregate functions

User-defined aggregate functions are now available. To create a
user-defined aggregate function, the 'aggregate' option should be set to
'group'.  The function must have at least one argument. The last
argument is always the result of the function in the previous step, or
NULL if it is the first step. The last argument will be added
automatically.

Example:
```
box.schema.func.create("F1", {language = "Lua",
    body = [[
        function(x, state)
            if state == nil then
                state = {sum = 0, count = 0}
            end
            state.sum = state.sum + x
            state.count = state.count + 1
            return state
        end
    ]],
    param_list = {"integer", "map"},
    returns = "map",
    aggregate = "group",
    exports = {"SQL"},
})

box.execute('CREATE TABLE t (i INT PRIMARY KEY);')
box.execute('INSERT INTO t VALUES(1), (2), (3), (4), (5);')
box.execute('SELECT f1(i) FROM t;')
```

The result will be:
```
tarantool> box.execute([[select f1(i) from t;]])
---
- metadata:
  - name: COLUMN_1
    type: map
  rows:
  - [{'sum': 15, 'count': 5}]
...
```

To create a finalizer for a function, another function should be
created. This function should follow the following rules:
1) its name should be '<function name>_finalize';
2) it must take exactly one argument;
3) it must be a non-aggregate function.

If an aggregate function has a finalizer, the result of the aggregate
function will be determined by the finalizer.

Example:
```
box.schema.func.create("F1_finalize", {
    language = "Lua",
    body = [[
        function(state)
            if state == nil then
                return 0
            end
            return state.sum / state.count
        end
    ]],
    param_list = {"map"},
    returns = "number",
    exports = {'LUA', 'SQL'},
})
box.execute('SELECT f1(i) FROM t;')
```

The result will be:
```
tarantool> box.execute([[select f1(i) from t;]])
---
- metadata:
  - name: COLUMN_1
    type: number
  rows:
  - [3]
...
```