postgresql@16: support extension formulae

[cho-m]

• [ ] Have you followed the guidelines for contributing?
• [ ] Have you ensured that your commits follow the commit style guide?
• [ ] Have you checked that there aren't other open pull requests for the same formula update/change?
• [ ] Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [ ] Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• [ ] Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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Another attempt at trying to get extensions working on other PostgreSQL formulae.

This time, keg directory structure is mostly unchanged other than datadir using #{share}/postgresql rather than #{share}/postgresql@16 for PostgreSQL 16, with a note to simplify some directory structure in PostgreSQL 17.

[p-linnane]

@cho-m is there anything else needed here?

[cho-m]

@cho-m is there anything else needed here?

I may hold off until postgresql@17 and only forward apply changes to reduce impact. May switch to beta testing until Sept/Oct release.

Mainly since I haven't sufficiently tested user scenarios and use cases. PostgreSQL 17 upgrade is essentially opt-in so should be able to handle any problems before larger adoption.

[9999years]

I've worked out (independently) the correct ./configure and make and make install-world args for this here, maybe we can integrate them: https://github.com/MercuryTechnologies/homebrew-tap/blob/0d611b7c9b3d261a855011a9bf7d8d3d11faa37b/Formula/postgresql%4016.rb

[cho-m]

Copied over existing work to #181862 to figure out something for postgresql@17.