Add pg-jsonschema extension target

[elohmeier]

Summary

• add a new pg-jsonschema extension target
• add build recipe in pg-jsonschema/Dockerfile to compile and install pg_jsonschema from upstream release tags
• add extension metadata in pg-jsonschema/metadata.hcl for PostgreSQL 18 on bookworm and trixie
• add usage documentation in pg-jsonschema/README.md
• add pg-jsonschema to the supported extensions table in README.md

Notes

• this target currently builds from upstream source tags (Supabase pg_jsonschema) rather than a PGDG package.

Validation

• docker buildx bake -f pg-jsonschema/metadata.hcl -f docker-bake.hcl --check

[NiccoloFei]

Hi @elohmeier, could you please open a new issue for pg_jsonschema following the New Extension Proposal template? Thanks.