Add sample for passing timeout and retry parameters

[asthamohta]

[CaliWee]

The documentation for configuring request timeouts is not well documented in Python -
https://cloud.google.com/spanner/docs/custom-timeout-and-retry

It seems that setting a timeout for a request is possible, as it is documented in the SpannerClient class - https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.services.spanner.SpannerClient

However, most use cases in the documentation use the Client, Instance, and Database class -
https://cloud.google.com/spanner/docs/reference/libraries#use
https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.client.Client

And from the Client, an instance and database class are instantiated to conduct operations.
Personally I have been attempting to utilize the transaction method batch_update -
https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.transaction.Transaction#google_cloud_spanner_v1_transaction_Transaction_batch_update

The documentation specifies that you may provide request options to this method, but the only option that is configurable is the Priority of the request.
https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.types.RequestOptions

In practice this method seems to have a default timeout of ~30s -

In the database.run_in_transaction method documentation there is a kwarg "timeout_secs", but this is the timeout configuration for the retry behavior and not the request itself. Setting this timeout to 540 does not resolve the request timeout issue.
https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.database.Database#google_cloud_spanner_v1_database_Database_run_in_transaction

As a user, it would be really helpful if the documentation were updated wherever possible to help users configure timeouts as the default timeout settings seem to vary between operations and can be limiting.