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.

[elohmeier]

Hi @NiccoloFei, thanks for the heads up - I've missed that issue template. Created the issue at #117.

[gbartolini]

I am not in favour of publishing extension images that need to be compiled. 👎🏻

The right approach is to get the extension packaged first.

[ibotty]

@gbartolini I understand your position. Should one try to get upstream to build packages first?

What should we do in case upstream does not want to?

[elohmeier]

Updated the Dockerfile to use prebuilt .deb packages from Supabase's GitHub releases instead of compiling from source. pg_jsonschema isn't available in PGDG — would Supabase's official release debs be acceptable?

[gbartolini]

Thanks @elohmeier. I think it is easier for you to close this PR and open a new one following the new process, as the metadata file is missing a lot of relevant information, such as licensing (which is now extremely critical as we are distributing these images as CNCF and need to comply with their policies).