Feed attributes from live_view declarative assigns

[cblavier]

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Upcoming liveview 0.18.0 will ship with the ability to declare component assigns.

phx_live_storybook Attr API is intentionally very close with declarative component assigns, so the idea is that:

• if using liveview 0.17 or less, only storybook attributes are used
• if using liveview 0.18+, then component attributes and storybook attributes are merged (storybook ones have higher priority)