[YSQL][backup] Backup-restore failed due to unexpected NULLABLE column attribute.

[OlegLoginov]

YB implementation for SQL statements CREATE TABLE and ALTER TABLE ADD can set internal NULLABLE attribute differently. As result the column attributes can be different if it was created via CREATE TABLE and via ALTER TABLE ADD.

Reproducible scenario:

1. CREATE TABLE test_tbl(id int);
2. ALTER TABLE test_tbl ADD a int NOT NULL;
3. Create backup for the DB.
4. Restore the backup into a new DB. Error:

Error: Internal error (yb/master/catalog_manager_ent.cc:1157): Unable to import snapshot meta file /tmp/yb_backup_mlplorihrpnhczll/SnapshotInfoPB: Invalid created table schema={Schema [
	10:ybrowid[binary NOT NULL PARTITION KEY],
	11:id[int32 NULLABLE NOT A PARTITION KEY],
	12:a[int32 NULLABLE NOT A PARTITION KEY],
]
properties: contain_counters: false is_transactional: true consistency_level: STRONG use_mangled_column_name: false num_tablets: 6 is_ysql_catalog_table: false retain_delete_markers: false}, expected={Schema [
	10:ybrowid[binary NOT NULL PARTITION KEY],
	11:id[int32 NULLABLE NOT A PARTITION KEY],
	12:a[int32 NOT NULL NOT A PARTITION KEY],
]
properties: contain_counters: false is_transactional: true consistency_level: STRONG use_mangled_column_name: false num_tablets: 6 is_ysql_catalog_table: false retain_delete_markers: false}: SNAPSHOT_FAILED (master error 23)

In the example ALTER TABLE ADD created a column with internal attribute 'NOT NULL. But on the backup-restore step the same column is created via CREATE TABLE with attribute NULLABLE`.

[OlegLoginov]

Fix:

1. ALTER TABLE ADD and CREATE TABLE statements must add columns using the same logic for the column attributes.
2. For backward compatibility backup-restore should ignore the 'nullable' column attribute to be able restoring from backups created by older releases.