Add a property to .info files to allow modules to specify required php extensions

It's fairly clear that composer is the way forward here. Contrib modules can already declare their PHP dependencies in composer.json. For core, the granularity is nice, but I think we can just add to the core composer.json if extensions are required. Are we happy to close this?

See also:
✨ Replace .info.yml with composer.json for extensions Postponed
#3005229: Provide optional support for using composer.json for dependency metadata →

When composer is on a different server like ci you can run into issues.

I ran into an issue a while back where I installed search api and it had a php requirement I had locally in ddev and in the CI tooling but not production so the site broke, this key would have prevented it. Most modules use hook requirements though for this a key might be easier.