Support AVG for durations #15458

> create or replace table t(user_id int, ts timestamp) as values (1, '2021-01-01'), (2, '2021-01-02'), (1, '2021-01-03'), (2, '2021-01-04');
0 row(s) fetched.
Elapsed 0.003 seconds.

> select * from t;
+---------+---------------------+
| user_id | ts                  |
+---------+---------------------+
| 1       | 2021-01-01T00:00:00 |
| 2       | 2021-01-02T00:00:00 |
| 1       | 2021-01-03T00:00:00 |
| 2       | 2021-01-04T00:00:00 |
+---------+---------------------+
4 row(s) fetched.
Elapsed 0.006 seconds.

> select avg('2021-01-01'::timestamp -  ts) from t;
Error during planning: Execution error: Function 'avg' user-defined coercion failed with "Error during planning: The function \"avg\" does not support inputs of type Duration(Nanosecond)." No function matches the given name and argument types 'avg(Duration(Nanosecond))'. You might need to add explicit type casts.
	Candidate functions:
	avg(UserDefined)

> select user_id, avg('2021-01-01'::timestamp -  ts) from t GROUP BY user_id;
Error during planning: Execution error: Function 'avg' user-defined coercion failed with "Error during planning: The function \"avg\" does not support inputs of type Duration(Nanosecond)." No function matches the given name and argument types 'avg(Duration(Nanosecond))'. You might need to add explicit type casts.
	Candidate functions:
	avg(UserDefined)

postgres=# create table t(user_id int, ts timestamp);
CREATE TABLE
postgres=# insert into t  values (1, '2021-01-01'), (2, '2021-01-02'), (1, '2021-01-03'), (2, '2021-01-04');
INSERT 0 4
postgres=# select avg('2021-01-01'::timestamp -  ts) from t;
        avg
-------------------
 -1 days -12:00:00
(1 row)

postgres=# select user_id, avg('2021-01-01'::timestamp -  ts) from t GROUP BY user_id;
 user_id |   avg
---------+---------
       2 | -2 days
       1 | -1 days
(2 rows)