feat: generate default clause when table column NULL is changed

[gurminder71]

When either a NOT NULL clause on a table column is changed, both NOT NULL and any DEFAULT clauses need to appear in the ALTER TABLE ALTER COLUMN statement, otherwise DEFAULT is dropped.

eg:

# original: 
CREATE TABLE test (
  id STRING(36),
  enforcement_enabled BOOL DEFAULT (TRUE),
) PRIMARY KEY (id)

# new
CREATE TABLE test (
  id STRING(36),
  enforcement_enabled BOOL NOT NULL DEFAULT (TRUE),
) PRIMARY KEY (id)

# diff needs both NOT NULL and DEFAULT
ALTER TABLE test ALTER COLUMN enforcement_enabled BOOL NOT NULL DEFAULT (TRUE)