sql/stats: inaccurate avg_size in statistics for column in single-column column family

Single-column column families are encoded using legacy marshaling. When we collect statistics on these single-column column families, we estimate their size [using Datum.Size() instead of the actual on-disk size](https://github.com/cockroachdb/cockroach/blob/88b6c94709222269d0bb75ef9be555cc7e554dad/pkg/sql/rowenc/encoded_datum.go#L116), and this can be very inaccurate. Here's an example:

```sql
CREATE TYPE e AS ENUM ('hello', 'howdy', 'hi');
CREATE TABLE abc (a e NOT NULL, b e NOT NULL, c e NOT NULL, FAMILY (a, b), FAMILY (c));
INSERT INTO abc VALUES ('hi', 'hi', 'hi');
ANALYZE abc;
SHOW STATISTICS FOR TABLE abc;
```

The avg_size of both `a` and `b` is 3 bytes, but avg_size of `c` is 48 bytes because DEnum.Size() includes a bunch of metadata that isn't written to disk.

```
demo@127.0.0.1:26257/demoapp/defaultdb> SHOW STATISTICS FOR TABLE abc;
statistics_name | column_names | created | row_count | distinct_count | null_count | avg_size | partial_predicate | histogram_id | full_histogram_id
------------------+--------------+-------------------------------+-----------+----------------+------------+----------+-------------------+---------------------+--------------------
NULL | {a} | 2024-12-18 17:25:46.427418+00 | 1 | 1 | 0 | 3 | NULL | 1030462984780972033 | 0
NULL | {b} | 2024-12-18 17:25:46.427418+00 | 1 | 1 | 0 | 3 | NULL | 1030462984784281601 | 0
NULL | {c} | 2024-12-18 17:25:46.427418+00 | 1 | 1 | 0 | 48 | NULL | 1030462984787001345 | 0
NULL | {rowid} | 2024-12-18 17:25:46.427418+00 | 1 | 1 | 0 | 9 | NULL | 1030462984777924609 | 0
(4 rows)
```

Jira issue: CRDB-45715