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.