Add AlloyDB vector writer. (#33841) (#33880)

* 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.

Co-authored-by: claudevdm <33973061+claudevdm@users.noreply.github.com>

Author: damccorm