isEditedPostDateFloating should be filterable to support custom statuses

[davisshaver]

Is your feature request related to a problem? Please describe.
Working with the Edit Flow project, we have run into an issue where posts with custom statuses are unable to stay in "floating" date mode.

See discussion here: Automattic/Edit-Flow#623

Describe the solution you'd like
Allow the list of statuses supporting floating date mode to be filtered.

Current

const status = getEditedPostAttribute( state, 'status' );
if (
    status === 'draft' ||
    status === 'auto-draft' ||
    status === 'pending'
) {
    return date === modified || date === null;
}
return false;

Proposed

const status = getEditedPostAttribute( state, 'status' );
const floatingStatuses = applyFilters(
	'selectors.getPostDateFloatingStatuses',
	[ 'draft', 'auto-draft', 'pending', ],
);
if (floatingStatuses.includes(status)) {
	return date === modified || date === null;
}
return false;

Describe alternatives you've considered
Disabling Edit Flow or adding a notification letting users know they need to work around the Gutenberg limitation.

Related to:

• Custom statuses are not supported #3144
• Save button label should be filterable to better enable custom workflows with custom statuses #18743

[davisshaver]

@talldan Thanks for the labels! Maybe [Feature] Extensibility too?