Add generator for PHP

[akatasonov]

Would love to see a PSR7 compatible API generator for PHP. Would be glad to create one if I only knew Scala...

[gheine]

@akatasonov, would be great to see more languages adopt API Builder! And good news, the generators don't need to be implemented in Scala at all! All that's required is that the generator implements the apibuilder-generator API, see https://app.apibuilder.io/apicollective/apibuilder-generator/latest
It's completely up to you which language or framework you use to implement it, and in fact where you host/run it. There is no requirement that the code generator is hosted on apibuilder.io.

[akatasonov]

Great news, @gheine! That's certainly something I can work with.