Add AlloyDB vector writer. (apache#33841)

* Add AlloyDB vector writer.

* Resolve comments.

* Add default ConflictResolution as ON CONFLICT DO NOTHING.

* Disable tests.

* Fix docstring errors.

* Simplify ColumnSpec's usage, add builder class to make it obvious what is default and how to override defaults or go full custom.

* Fix linter error.

Author: claudevdm