Add mu-plugins loading filter

[dd32]

There are some occasions where having the ability to filter the mu-plugins being loaded would be useful.

Regular plugins and network activated plugins can be filtered through option filters, but the mu-plugins can't, as no option exists.

In my case, I'd like to avoid loading any plugin content via a sunrise.php file, before a wp-env environment is fully configured, but without having to have to start the environment without mappings and then add them in.

The attached PR adds filters to wp_get_mu_plugins(), wp_get_active_network_plugins(), wp_get_active_and_valid_plugins(), and wp_ get_active_and_valid_themes() for good measure.

Trac ticket: https://core.trac.wordpress.org/ticket/64665

## Use of AI Tools

[jorbin]

Some previous related discussion: #34936

First note is that documentation similar to what is on functions such as enable_wp_debug_mode_checks and enable_loading_advanced_cache_dropin explaining that these filters run before plugins are loaded would be helpful.

For the regular plugins and network activated plugins, I think filtering on the option makes more sense for two reasons:

1. When removing plugins, you avoid the wasted calls to things such as validate_file.
2. When adding plugins, you get the benefit of things such as the network_plugins collision prevention and recovery mode.

In my case, I'd like to avoid loading any plugin content via a sunrise.php file, before a wp-env environment is fully configured, but without having to have to start the environment without mappings and then add them in.

To avoid loading any mu-plugins, you can always define WPMU_PLUGIN_DIR to a non directory in your sunrise.php. But this is hacky and an actual extension point makes better sense.

Nitpicking: is there a reason the wp_get_active_network_plugins filter has the wp_ prefix, while the others don't?