Spring Data Cloud Spanner missing @ReadOnlyProperty support

spring-cloud-gcp-starter-data-spanner 5.11.2

Spring Data Cloud Spanner does not seem to have support for allowing a field of an entity to be populated from query results but prevent it from being written to the database.

This capability is needed for fields that are derived from queries (e.g., computed columns or subquery results) and should not be modified or saved.

Spring Data has the @ReadOnlyProperty which does just that and should be a supported feature.

Example
```
SELECT
foo.*,
EXISTS (
SELECT 1
FROM foo as bar
WHERE bar.external_id = foo.id
) as has_bar
FROM
foo
WHERE
foo.name = @fooName
```

In this case we want the data mapping to populate `has_bar` on our entity when issuing a query but to ignore `has_bar` when inserting or updating the entity (since has_bar is not a real column).