Header: Allow App Blocks

[nicklepine]

User story
As a merchant, I can add app blocks to my header.

Context
Given the header section needs to be broken down into blocks in order to support app blocks, we'll need to revisit the design and how we broke down the settings for its different components. This means a larger design effort than other already "blockified" sections.

[phangjunyu]

hello! Could you share how you managed to add app blocks to your header section? Been trying really hard to figure this out