fix: Update how types are exported

[eablack]

Currently, importing pg types is a little bit broken, requiring to import directly from dist as shown here: https://github.com/heroku/cli/blob/v11.0.0/packages/cli/src/lib/addons/resolve.ts#L6. This PR improves how we export/organize the types using namespace so that they can more cleanly be imported like:

import type { pg } from '@heroku/heroku-cli-util'
...
const connection: pg.ConnectionDetailsWithAttachment = { ... }

[michaelmalave]

@eablack Changes look good! I wonder if the README should also be updated too to track this updated import of types since they're mentioned there?

[michaelmalave]

LGTM!