Support timestamp functions

[hamoelze]

Hello, On hammer apply or hammer diff, while creating a table with a generated column using a timestamp function, such as:

CREATE TABLE examples (
  inserted_at TIMESTAMP  NOT NULL,
  examples_id STRING(36) NOT NULL,
  examples_timestamp TIMESTAMP  AS(TIMESTAMP_ADD(inserted_at, INTERVAL 1 DAY)) STORED
) PRIMARY KEY(examples_id);

produces the following error: Error: spanner://projects/test-project/instances/test-instance/databases/example_dev failed to parse ddl: spanner://projects/test-project/instances/test-instance/databases/example_dev:4: got "1", want ")" or ","

[neglect-yp]

related issue: https://github.com/googleapis/google-cloud-go/issues/6949