react-email: Image lacks className and there's no way to link an image

Two gaps in the standard email components that force custom overrides:

1. Image has no className (catalog.ts#L154). Responsive emails rely on a <style> media query targeting a class (e.g. stack columns / full-width image on mobile), so without it there's no way to make images responsive.
2. No linked image. Image has no href, and Link only renders props.text (catalog.ts#L118), so a clickable logo or banner (very common in email) can't be expressed with standard components.

Proposal: add optional className to Image (and Column/Section), and an optional href on Image that wraps it in <Link>.

Both are small, happy to send a PR.