feat: Support INTERVAL type

[apstndb]

This PR implements INTERVAL type decoding.

I understand that this project is currently in maintenance mode, but I believe that unsupported types represent a noteworthy issue that cannot be ignored for some users.

• Closes Support INTERVAL type #213

Before

$ go run github.com/cloudspannerecosystem/[email protected] -v -e 'SELECT INTERVAL 1 DAY, [INTERVAL "8 20 17" MONTH TO HOUR]'
+--------------------+-----------------+
|                    |                 |
| INTERVAL           | ARRAY<INTERVAL> |
+--------------------+-----------------+
| string_value:"P1D" | []              |
+--------------------+-----------------+
1 rows in set (0.2 msecs)
timestamp:            2026-01-31T04:01:32.949479+09:00
cpu time:             0.18 msecs
rows scanned:         0 rows
deleted rows scanned: 0 rows
optimizer version:    8
optimizer statistics: auto_20260129_12_27_00UTC

After

$ go run ./ -v -e 'SELECT INTERVAL 1 DAY, [INTERVAL "8 20 17" MONTH TO HOUR]'
+----------+-----------------+
|          |                 |
| INTERVAL | ARRAY<INTERVAL> |
+----------+-----------------+
| P1D      | [P8M20DT17H]    |
+----------+-----------------+
1 rows in set (2.55 msecs)
timestamp:            2026-01-31T03:37:19.656654+09:00
cpu time:             2.52 msecs
rows scanned:         0 rows
deleted rows scanned: 0 rows
optimizer version:    8
optimizer statistics: auto_20260129_12_27_00UTC