Add timestamp of export as internal option to export

[sun]

Problem

• When working with a clean copy of a database, I don't know how old the configuration and content is.

Proposed solution

1. When creating the database export, add a line to insert a hidden new internal option field that holds the date of the export (hardcoded, generated at time of export); e.g.:

INSERT INTO wp_options (option_name, option_value, autoload) VALUES ('wp-cli-db-export-clean/date', '2025-06-10 17:35', 'no');