Global settings?

It’s technically possible, but probably too complex/nuanced.

This will add the data-pin-description and data-pin-title values for core/image blocks by default.

add_action('enqueue_block_editor_assets', function() {
	$js = <<<JS
wp.hooks.addFilter('blocks.registerBlockType', 'attributes-for-blocks/defaults', settings => {
	if(settings.name === 'core/image') {
		return {
			...settings,
			attributes: {
				...settings.attributes,
				attributesForBlocks: {
					...settings.attributesForBlocks,
					default: {
						'data-pin-description': '',
						'data-pin-title': '',
					},
				},
			}
		}
	}
	return settings
});
JS;
	wp_add_inline_script('attributes-for-blocks', $js);
});

It will invalidate any existing blocks that don’t have custom attributes as it now has a different default value.

should it be added to one of your existing plugin files or to one of the core WP files

Your child theme’s functions.php is the preferred location. If you add to plugin or WP core files the changes get overwritten on updates.

Is it possible to add these attributes to the IMG tags – your suggested code adds attributes to FIGURE tags

No, the plugin can only add attributes to the root element blocks.

I may add these features you’re looking for in the future, but currently I don’t have the time as I’m working on something else, sorry.