Need Dependency Validator Util

It would be cool to create a util that can be re-used to "validate helper dependencies". For helpers that need to check if a plugin is installed or not, it would be cool to have a standard approach to this.

```
class DependencyValidator {

static function our_normal_plugin_checker( $plugin_name ) {
if ( ! function_exists( 'is_plugin_active' ) ) {
require_once ABSPATH . 'wp-admin/includes/plugin.php';
}

if ( ! is_plugin_active( $plugin_name ) ) {
return new WP_Error( 'ERROR_PLUGIN_MISSING', 'Plugin not found. Install and activate it before using this function.' );
}
}

static function validate_co_authors_plus() { ... }
static function validate_newspack_guest_contributors() { ... }
}
```

The current code for all this is spread out in the different helpers ( [example](https://github.com/Automattic/newspack-migration-tools/pull/130), [example](https://github.com/Automattic/newspack-migration-tools/blob/42a9f29772e3b564f8bf54746318e576cdfb46dd/src/Logic/GuestContributorsHelper.php#L278) ) , but we should put this into a sharable Util so different helpers that need the same plugin can just call the shared validator.

Start with src/Logic/GuestContributorsHelper.php around the dependency-checking code referenced in the issue, and compare it with the examples in pull request 130. Identify the repeated plugin validation used by the helpers; done means a shared validator exists and the relevant helpers can reuse it consistently.

Written by the indexing model from the issue text.