Support for foreign key/joins in Spring Data Cloud Spanner

**Is your feature request related to a problem? Please describe.**
The [documentation](https://cloud.spring.io/spring-cloud-gcp/1.1.x/multi/multi__spring_data_cloud_spanner.html) for Spring Data for Cloud Spanner claims that foreign key constraints are not natively supported in cloud spanner. Yet according to [this documentation](https://cloud.google.com/spanner/docs/foreign-keys/overview#how-to-define-foreign-keys), they are supported. While it seems interleaving is the ideal solution for one-to-many relationships, foreign keys are necessary for many-to-many relationships and sometimes for one-to-one. However it seems to ORM mapping annotations for spanner only supports `@Interleaved`. What's the current suggested solution for mapping `@OneToOne` and `@ManyToMany` relationships?

**Describe the solution you'd like**
Adding `@OneToOne`, `@OneToMany`, `ManyToOne`, and `@ManyToMany` mapping annotations to the [com.google.cloud.spring.data.spanner.core.mapping](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/tree/main/spring-cloud-gcp-data-spanner/src/main/java/com/google/cloud/spring/data/spanner/core/mapping) package for cases where we want to use foreign keys rather than interleaving.