Adding italics to block template

I’m trying to register a block template using PHP. I need to be able to add modifiers to the block. For example, adding bold or italics to a heading block. My code below works, I am just unable to find out how to add strong or italic to the template. Can anyone help?

My current block template:

array(
    'core/heading',
	array(
		'level'       => 1,
		'content' => 'This is the page title',
	),
),

I have tried things like the following (strong = true):

array(
    'core/heading',
	array(
		'level'       => 1,
		'content' => 'This is the page title',
		'strong' => true,
	),
),

But this does not work. Any help?