Find and close all SpannerClientLibraryConnectionFactory on Spring Data context shutdown

[elefeint]

1. Pending outcome of R2DBC ConnectionFactory bean .close() method should be called/subscribed when available spring-projects/spring-framework#26991, add a listener for ContextClosedEvent to find SpannerClientLibraryConnectionFactory beans and call Closeable.close().subscribe(). Might have to even block to avoid race conditions ("app shut down before sessions are released")

2. Add stronger documentation warnings about having to close custom created Spanner connection factories on context shutdown (to avoid server-side session leaks, as the client library pre-allocates ~200 sessions).