spannertest: add support for row deletion policy

[alethenorio]

Is your feature request related to a problem? Please describe.
We use spannertest to build an inmemory database based on a set of DDLs however as spannertest does not have any support for row deletion policy, it will fail calls to UpdateDDL with unhandled DDL table alteration type spansql.AddRowDeletionPolicy.

Describe the solution you'd like
The simplest solution would be to allow row deletion policy DDLs to not fail UpdateDDL calls (just ignore the row deletion policy) which would allow for performing basic functionality such as querying and other update

Describe alternatives you've considered
Alternative solutions could be to add actual support for row deletion policy (although given that the minimum is 1 day I am not sure how useful it is given the scope of spannertest). A configurable option allowing the user to choose to ignore row deletion policy and not fail UpdateDDL calls could also be an option.

Additional context

[hengfengli]

@olavloite Can you please triage this issue? Thanks.

[olavloite]

@alethenorio Do you have an example of a DDL statement with a row deletion policy that does not work for you? I had a second look into this today, and it turns out that it is implemented:

1. Parsing:

   google-cloud-go/spanner/spansql/parser.go

   Lines 1087 to 1093 in 2c22a16

   	if p.eat(",", "ROW", "DELETION", "POLICY") {
   	rdp, err := p.parseRowDeletionPolicy()
   	if err != nil {
   	return nil, err
   	}
   	ct.RowDeletionPolicy = &rdp
   	}

2. Tests for both creating and dropping:

   google-cloud-go/spanner/spansql/parser_test.go

   Lines 492 to 501 in 2c22a16

   	-- Table with row deletion policy.
   	CREATE TABLE WithRowDeletionPolicy (
   	Name STRING(MAX) NOT NULL,
   	DelTimestamp TIMESTAMP NOT NULL,
   	) PRIMARY KEY (Name)
   	, ROW DELETION POLICY ( OLDER_THAN ( DelTimestamp, INTERVAL 30 DAY ));
   	ALTER TABLE WithRowDeletionPolicy DROP ROW DELETION POLICY;
   	ALTER TABLE WithRowDeletionPolicy ADD ROW DELETION POLICY ( OLDER_THAN ( DelTimestamp, INTERVAL 30 DAY ));
   	ALTER TABLE WithRowDeletionPolicy REPLACE ROW DELETION POLICY ( OLDER_THAN ( DelTimestamp, INTERVAL 30 DAY ));

[olavloite]

@alethenorio Friendly ping. Do you have an example of a statement with a row deletion policy that does not work?

[alethenorio]

@olavloite Thanks for the ping. Missed the first time around.

The issue is when calling this API

google-cloud-go/spanner/spannertest/inmem.go

Line 270 in 25cd7a1

func (s *Server) UpdateDDL(ddl *spansql.DDL) error {

Which eventually ends up in this switch statement

google-cloud-go/spanner/spannertest/db.go

Line 257 in 25cd7a1

switch stmt := stmt.(type) {

Which has a separate logic from spansql and does not support row deletion policies and instead return the default case of the switch.