feat: add pg-jsonschema container image

[elohmeier]

Description

This PR adds a pg-jsonschema extension container image for PostgreSQL 18 on
Debian bookworm and trixie.

pg_jsonschema provides JSON Schema validation for PostgreSQL json and
jsonb data.

Packaging note: pg_jsonschema is currently not available through the PG 18
bookworm or trixie base-image apt sources. This image intentionally
installs Supabase's official GitHub release .deb assets from:

https://github.com/supabase/pg_jsonschema/releases/tag/v0.3.4

The package contains pg_jsonschema.control, pg_jsonschema--0.3.4.sql, and
pg_jsonschema.so. The final image remains FROM scratch and copies only the
extension artifacts plus the upstream Apache-2.0 license.

Type of change

• Update to an existing extension (version bump, fix)
• Shared build infrastructure / tooling change
• Documentation only
• Other (please describe above)

Closes #117. Follow-up to #103.

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Contributor checklist

• My commits are signed off for DCO
  compliance (git commit -s).
• This PR targets the main branch of the upstream repository.
• task checks:all passes locally.
  Not run locally.
• For changes affecting one or more extensions, the relevant build and E2E
  tests pass (e.g. task bake TARGET=<ext>, task e2e:test:full TARGET=<ext>).
  task bake TARGET=pg-jsonschema passed locally; full E2E was not run
  locally.
• Any // renovate: comments touched in metadata.hcl / README.md remain
  intact (suite=, depName=, extractVersion=).
  This PR uses GitHub tag-based Renovate annotations for
  supabase/pg_jsonschema.
• Documentation (README.md, BUILD.md, ...) updated where relevant.

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Maintainer review checklist

• CI is green for all affected targets / shared changes.
• DCO check passes.
• Change reviewed for correctness and scope; no unintended targets rebuilt.
• // renovate: annotations preserved so automated version tracking keeps
  working.
• If shared infrastructure changed, the _shared filter in
  .github/workflows/bake.yml was updated when all extensions must rebuild.
• PR targets main and is ready to merge.