Support Default on Google Cloud Spanner

[zaitera]

Is your feature request related to a problem? Please describe.
We can't run migrations with DEFAULT (for example adding column and setting a default value for the existing record)

Describe the solution you'd like
Being able to execute statements like the following:
ALTER TABLE t1 ADD COLUMN c1 STRING(MAX) DEFAULT ('');
Google cloud spanner already supports this syntax, and I've tried it on the cloud console

Describe alternatives you've considered
Disabling CleanStatements makes it possible to execute that statement, but then we lose the option to have comments or multiple statements in the migration file

Additional context
Add any other context or screenshots about the feature request here.

[IDispose]

This will be more useful if the following is also supported
ALTER TABLE t1 ADD COLUMN c1 STRING(MAX) NOT NULL DEFAULT ('')

Currently this throws an error saying Cannot add non-null columns to an existing table

[IDispose]

For anyone looking for a workaround, this works.

ALTER TABLE t1 ADD COLUMN c1 STRING(MAX) DEFAULT ('');
ALTER TABLE t1 ALTER COLUMN c1 STRING(MAX) NOT NULL DEFAULT ('')

put those together and your table DDL will look like this

CREATE TABLE 
    t 1 (
        -- other columns
        c1 STRING(MAX) NOT NULL DEFAULT ('')
    )