Aggregate function sum() not supported on a varint or decimal column

[rkarthik007]

Test case:

cqlsh> create keyspace if not exists k;
cqlsh> use k;
cqlsh:k> create table t (k int primary key, n decimal);
cqlsh:k> insert into t (k, n) values (1, 1);
cqlsh:k> insert into t (k, n) values (2, 2);
cqlsh:k> select * from t;

 k | n
---+---
 1 | 1
 2 | 2

(2 rows)
cqlsh:k> select sum(n) from t;
InvalidRequest: Error from server: code=2200 [Invalid query] message="SQL error (yb/yql/cql/ql/ptree/process_context.cc:51): Invalid Function Call. Failed calling 'sum(decimal)'. Not implemented (yb/util/bfql/bfql.cc:228): Builtin function 'sum' is not yet implemented
select sum(n) from t;
       ^^^
 (error -214)"

Reported by @robertpang

[rven1]

Landed https://phabricator.dev.yugabyte.com/D5065