Add filter for `get_mu_plugins()`

[rebasaurus]

In https://core.trac.wordpress.org/ticket/57278, there was a need to be able to filter MU-plugins on the UI. However, for WP-CLI, when we call wp plugin list for MU-plugins, it calls get_mu_plugins(). It'd be nice if it was filterable to match the changes on the UI end.

It does look like WP-CLI does have the filter all_plugins for the drop-in plugins at ​https://github.com/wp-cli/extension-command/blob/17b16548b5775616dcbbd92f7836e67bba02e8ba/src/WP_CLI/Fetchers/Plugin.php#L23 to match Core's at ​https://github.com/WordPress/WordPress/blob/2a2cbb3b497fa1b95f15b0d11a47a761fa2ccc21/wp-admin/includes/class-wp-plugins-list-table.php#L104, but that appears to only apply for the drop-ins.