Pattern Overrides: Refactor block toolbar indicator

[talldan]

See #65054
Related #62514

Inner blocks of patterns currently use a custom filter to add a block icon to the toolbar. See this code:

gutenberg/packages/patterns/src/components/pattern-overrides-block-controls.js

Lines 150 to 153 in 559edae

	return hasPatternOverrides && hasParentPattern ? (
	<BlockControls group="parent">
	<PatternOverridesToolbarIndicator clientIds={ clientIds } />
	</BlockControls>

It was implemented this way in #62514 to try to extract the pattern specific code from the block editor package. It also worked since the pattern block leveraged contentOnly mode where selected blocks didn't have a block icon. That is until #64694 added a block icon for contentOnly blocks.

It's pretty clear now that the abstraction for pattern overrides needs to be changed, so that rather than adding a custom icon, the normal block icon is used instead. Here's the tasks:

• Remove the filter approach and instead use the regular block icon in the toolbar, revert De-duplicate block toolbar icons for patterns #65054 in the process
• Find a way to make the overridden block icons have the purple color, maybe a declarative way to mark the overridden blocks as 'synced'.
• Find a way to display this message in the block switcher dropdown, or reconsider whether it's needed:

[jeryj]

One direction to start with this is to separate out the block switcher from the simple icon. It has bothered me that the toolbar icon is powered by the block switcher and has the classname "block-editor-block-switcher__no-switcher-icon" when it is an icon indicator that does nothing.

I'd go in a direction that treats the indicator icon as the first case and then passes it to the switcher if we need to load a switcher rather than having them all combined in one component. I wouldn't mind having the block indicator be exported as well so it can be used elsewhere.