Install php with asdf

What does this MR do?

Commits in this MR are individually reviewable

Installs PHP with asdf, so we can pin the version:

It requires postgresql to link to libpq. By default the plugin is not smart enough to get this from the postgresql installed with asdf. We would have to brew install postgresql for it to work by default (not postgresql@xx), which means we couldn't pin the postgres version. I decided to monkey patch the plugin and give it the right path for postgres manually.

Why was this MR needed?

All other languages are installed and pinned with asdf

What's the best way to test this MR?

Start a VM from an image of this MR and execute php --version

What are the relevant issue numbers?

Closes #4 (closed)