feat: add plpgsql proto support

[psteinroe]

This pull request was written alongside Claude Code. As a fellow os maintainer, I can completely understand if it is closed without comments.

Adds support for parsing plpgsql into protobuf, replacing the experimental json output. I do not have much experience in C or Ruby but tried to follow existing patterns as close as possible. I verified it by pointing the Rust binding in the Postgres Language Server project at my fork. Tests pass, and it seems to work just fine (supabase-community/postgres-language-server#639).

This would enable much better plpgsql support in the Language Server! :)

Summary of changes:

• Add pg_query_parse_plpgsql_protobuf() API function
• Generate PLpgSQL types in pg_query.proto
• Add automated code generation scripts for PLpgSQL nodes
• Include outfuncs/readfuncs for serialization

[lfittl]

@psteinroe Thanks for the contribution, and appreciate the proactive disclosure of having used Claude Code - I think that's not a deal breaker as long as you have tested/reviewed the result.

I've been looking at this already as part of the Postgres 18 PR - I will take your work here into account (and may use parts of it), and will try to take this into the Postgres 18 update that I'm actively working on completing.

[lfittl]

@psteinroe I ended up running out of time to get this into the initial 18 release, but I think this still makes sense to do.

In case you have time available to work on this in this PR after all, two things that'd be helpful:

1. Rebase on Postgres 18 (against 18-latest branch)
2. Extend this to also auto-generate the JSON output functions, so we can address fix: serialize retvarno field in PLpgSQL RETURN/RETURN NEXT JSON output #333 as well