Skip dumping generated columns

[r3nic1e]

Generated columns cannot be inserted or updated so there is no reason to dump them. So DML should not contain fields for these columns

https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#create_table

Direct writes to generated columns are not allowed.

CREATE TABLE (
  Name STRING(256) NOT NULL,
  NameLow STRING(256) AS (LOWER(Name)) STORED,
  ...
);

ERROR: spanner: code = "InvalidArgument", desc = "Cannot INSERT value on non-writable column: NameLow