Default Partition for Range

Hello,

Many were in favour of the default partition for tables partitioned by
range [1].
Please find attached the WIP patch for the same which can be applied over
the default_partition_v12.patch.

Syntax: Same as agreed for list:
CREATE PARTITION <part_name> PARTITION OF <parent_tbl> DEFAULT;

Default Constraint:
Negation constraints of all existing partitions.

One case:
CREATE TABLE range1 (a int, b int) PARTITION by range (a);
CREATE TABLE range1_1 PARTITION OF range1 FOR VALUES FROM (1) TO (5);
CREATE TABLE range1_2 PARTITION OF range1 FOR VALUES FROM (7) TO (10);
CREATE TABLE range1_def PARTITION OF range1 DEFAULT;
\d+ range1_def
                                Table "public.range1_def"
 Column |  Type   | Collation | Nullable | Default | Storage | Stats target
| Description
--------+---------+-----------+----------+---------+--------
-+--------------+-------------
 a      | integer |           | not null |         | plain   |
 |
 b      | integer |           |          |         | plain   |
 |
Partition of: range1 DEFAULT
Partition constraint: (((a < 1) OR (a >= 5)) AND ((a < 7) OR (a >= 10)))

It still needs more work:
1. Handling addition of new partition after default, insertion of data, few
more bugs
2. Documentation
3. Regression tests

[1] Discussion on default partition:
*https://www.postgresql.org/message-id/flat/CAOgcT0PLPge%3D5U6%3DGU5SnC3_8yutCbWWOiUva3Cw94M9zpbvgQ%40mail.gmail.com
<https://www.postgresql.org/message-id/flat/CAOgcT0PLPge%3D5U6%3DGU5SnC3_8yutCbWWOiUva3Cw94M9zpbvgQ%40mail.gmail.com>*

-- 

Beena Emerson

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