Support non-nullable generated column

[toga4]

WHAT

Add support for inserting into table contains non-nullable generated columns.

WHY

Currently, handy-spanner gives an error if the inserting column is insufficient against the non-nullable columns

% handy-spanner &
[1] 38441
2022/10/15 15:13:04 spanner server is ready

% SPANNER_EMULATOR_HOST=localhost:9999 spanner-cli -p fake -i fake -d fake
Connected.
spanner> CREATE TABLE foobar (
      ->   id INT64 NOT NULL,
      ->   value STRING(MAX) NOT NULL AS (CAST(id AS STRING)) STORED,
      -> ) PRIMARY KEY(id);
Query OK, 0 rows affected (0.01 sec)

spanner> INSERT INTO foobar (id) VALUES (1);
ERROR: spanner: code = "FailedPrecondition", desc = "A new row in table foobar does not specify a non-null value for these NOT NULL columns: value"

This PR allows to insert without specifying columns with non-nullable but generated.

[kazegusuri]

Thank you for your contribution. Nice improvements and tests. It's enough to merge but there is a conflict. I added a commit to fix the conflict on top of your commits then merged your PR.