feat: add h3 container image

[jmealo]

What this adds

A container image for the core h3 PostgreSQL extension (h3-pg), which provides bindings for H3, Uber's hierarchical hexagonal geospatial indexing system. This enables CloudNativePG users to: index, aggregate, and join location data on a global hexagonal grid directly in SQL.

Closes #226

Details

• Package: postgresql-<pg-major>-h3 (PGDG)
• trixie: 4.2.3-4.pgdg13+1 · bookworm: 4.2.3-4.pgdg12+1
• SQL version: 4.2.3 (default_version in h3.control)
• PostgreSQL / distros: 18 on bookworm and trixie, linux/amd64 + linux/arm64
• Scope: ships only the self-contained core h3 extension. The optional h3_postgis companion (which requires PostGIS) is intentionally excluded.
• System library: h3.so dynamically links libh3.so.1 (from libh3-1, also PGDG). It is bundled under /system and exposed via ld_library_path = ["system"], mirroring the postgis image. auto_update_os_libs is enabled with system-libs/ manifests for renovate.

⚠️ License note (please review)

Both h3-pg and libh3/uber/h3 are Apache-2.0 upstream. However, the Debian libh3-1 package's copyright file classifies two upstream files (README.md, src/h3lib/lib/coordijk.c) as AGPL-3+, attributing them to DGGRID / Southern Oregon University. This looks like an over-conservative Debian classification of an algorithm-origin credit: current upstream uber/h3 carries Apache-2.0 headers on those files and contains no AGPL text. Because coordijk.c compiles into the bundled libh3.so.1 and the Debian copyright ships under /licenses/, it is surfaced for maintainer review
(also raised in #226).

Testing

• task e2e:test:full TARGET=h3 — passing: builds the multi-arch image, pushes to a local registry, deploys a CNPG Cluster with the h3 image volume + a Database, and asserts CREATE EXTENSION h3 (v4.2.3) applies cleanly.
• Multi-arch build (linux/amd64 + linux/arm64) and push verified for both bookworm and trixie.

Submission checklist

• Commit is DCO signed-off (git commit -s)
• PR targets the main branch of the upstream repository
• Adds a CODEOWNERS entry for /h3/
• Single squashed commit following the required feat: format