add META.json for PGXN distribution

[ardentperf]

It would be great to distribute pg_hint_plan on PGXN. In order to do that, we'll need a META.json file with the metadata needed by PGXN to catalog and manage the extension. This PR has a proposal.

few notes about this draft (open to discussion):

• intend not to commit any META.json to master branch but only to PG version branches from 11 to 16. Planning to use "requires": { "PostgreSQL": ">= 16.0.0, < 17.0.0" } for each one, and name the overall PGXN distro with the major version like “pg_hint_plan16". this matches the releases currently being done on github which are also titled like "pg_hint_plan16".
• this proposal puts the same pg_hint_plan version in both the PGXN distro and the PGXN package; we’d update both when we advance the pg_hint_plan version
• i think i did the right thing with the PGXN “file” tag, since ISTM that pg_hint_plan always runs through all the update to install the latest version
• PGXN requires a 'maintainer' name and email address in its metadata. AFAICT pg_hint_plan doesn't publicly list names of any individual maintainers at this time, and from what I can see the official way to contact maintainers is through GitHub issues. I used a dummy email that indicates this in the proposal here. however, if possible, it might be nice to list a few names in the META.json, if everyone agrees to have their name listed with an appropriate email address that can be used here.

once things are agreed on, I can create PRs for branches from v11 to v15. (or if github supports multiple branches in a single PR then i can do that, i just haven't tried this yet to see if it works.)

[ardentperf]

Another thought, it might be nice to have a checklist somewhere of all the places that need to be changed when advancing a version number. Could add a comment there to help make sure we don't accidentally miss any places. Seems like there are a number of files now that need version string updating when doing a new release.

[michaelpq]

FWIW, it is not a big deal to me to add something on HEAD as well. Actually, doing so has the advantage that we are unlikely going to forget about it when generating a new stable branch.

"name": "pg_hint_plan16",

Appending the version after the name is okay for me. That's what happens already in the docs and such.

Adding extra folks to gather more opinions: @rjuju @yamatattsu @mikecaat. Particularly, who do you think should be added to the list of maintainers? It would not be cool to add somebody without asking for permission first. I am OK to be in the list of that helps :)

[michaelpq]

So, hearing nothing, I have spent some time reminding myself how all this stuff works (like https://pgxn.org/meta/spec.txt), and applied that down to 12 by using my own name as maintainer. If you'd like to tweak the existing list, feel free.

I'll go upload some tarballs to pgxn in a bit.