Connectors: Add is_active callback support to plugin registration

[Adi-ty]

Backport of WordPress/gutenberg#76994. Adds an optional is_active callable to the plugin definition when
registering a connector via WP_Connector_Registry::register().

The Connectors screen previously resolved a connector's active/installed status by checking is_plugin_active() and file_exists() against WP_PLUGIN_DIR only. Plugins installed as must-use plugins — or loaded from non-standard paths could not be detected this way, causing their connectors to disappear from the screen even though the plugin was in use.

Trac ticket: https://core.trac.wordpress.org/ticket/65020

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[github-actions]

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