Add support for `ALTER SEARCH INDEX`

It appears the emulator does not support the [`ALTER SEARCH INDEX`](https://docs.cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#search_index_statements) syntax.

The following statement

```sql
ALTER SEARCH INDEX foo ADD COLUMN bar;
```

produces

```
Caused by: com.google.cloud.spanner.SpannerBatchUpdateException:
INVALID_ARGUMENT: io.grpc.StatusRuntimeException:
INVALID_ARGUMENT: Error parsing Spanner DDL statement:
ALTER SEARCH INDEX foo ADD COLUMN bar : Syntax error on line 1, column 7: Encountered 'SEARCH' while parsing: alter_statement
```

### Version
Image: 1.5.50 (958afe2419dd1)
https://console.cloud.google.com/artifacts/docker/cloud-spanner-emulator/us/gcr.io/emulator/sha256:958afe2419dd17c44f8205dfc63ad11ec00cbca3a8135bc3effde46a71dbc4e8