Render newsletter sign-up cards in Highlights using newsletter field

**Context:**
Highlights containers on fronts need to display newsletter sign-up cards when the card's pressed data supplies newsletter metadata.

**Tasks:**
- Add an optional `newsletter` field to FEFrontCard/DCRFrontCard types and schema.
- In `enhanceCards`, map newsletter data from FE to DCR card shape.
- In ScrollableHighlights, if `trail.newsletter` exists, render a new NewsletterHighlightsCard with supplied metadata (identityName, name, description, frequency, illustration, etc). Otherwise render a normal HighlightsCard.
- Match the look and dimensions of other highlights cards.
- Reference frontend issue for source data.

_See: https://github.com/guardian/email-rendering/issues/948_