Add an annotation for automatically adding pending_commit_timestamp()

[olavloite]

Cloud Spanner supports setting the value of a column to the commit timestamp of a transaction. Users must currently configure and handle this manually. Having an annotation that does this automatically would improve the user-friendliness of this dialect.

The annotation should:

1. Make sure the allow_commit_timestamp=true option on the column is set.
2. Automatically assign the commit timestamp for inserts.
3. Automatically assign the commit timestamp for updates.

2 and 3 should be separately configurable.