Feature / add consentManager Placeholder component

[enguerranws]

I added the consentManager's Placeholder component based on the example given on the stories.

While adding the consent manager on our project, the CI failed on accessbility tests (aXe). I took the liberty to add titleAs so we have not to stick to arbitrary H levels on the DSFR examples (which are, depending on the context, accessibility issues):

• replaced consentManager title by span, which won't break title hierarchy
• Placeholder title with titleAs prop (which default to h4 - same behavior as the dsfr demo)
• replaced consentManager modal wrapper tag by dialog (instead of div, to resolve missing landmarks issues)

Still discussing on it on Mattermost: https://mattermost.incubateur.net/betagouv/threads/jnzhstsadbdjib9ithqhotte7h

[enguerranws]

Still having issues:

• dialog seems to applies other styles (centered, thick border)
• cookies banner and consent management modal have the same ID

I think I'll remove all semantic / accessbility related things in another PR.

[garronej]

Let me know when you want me to review it!
And thanks a lot fot this!

[enguerranws]

@garronej feel free to review it :)

[garronej]

Looks great, thanks a lot!

[garronej]

<3