CrateDB: Respect special DEFAULT keyword in INSERT statements

[amotl]

Issue description

CrateDB needs to support the DEFAULT special keyword, like PostgreSQL does.

Expected Behavior

CrateDB supports such an invocation of SQL statements.

CREATE TABLE t (a INTEGER DEFAULT 5);
INSERT INTO t VALUES (DEFAULT);

• See also: Support DEFAULT keyword in INSERT statements crate/crate#14575

Actual Behavior

CrateDB currently lacks corresponding PostgreSQL feature.

/cc @hlcianfagna