feat: Add php

[markphelps]

This PR adds our PHP server SDK to the monorepo

It copies the majority of the code from https://github.com/flipt-io/flipt-php as well as the following changes:

• Updates to use authentication strategy and support JWT Auth
• Updates PHP tests to use integration style tests like other sdks

TODO:

• Still need to figure out the packaging as PHP / Packages requires a single repo per package so we'll likely need to sync the code to the existing repo: https://github.com/flipt-io/flipt-php. This is similar to what we have to do for the Go client-side SDK: https://github.com/flipt-io/flipt-client-sdks/blob/17d52077cb3f27477fb2fac3cca954cacfea8e84/build/main.go#L136

Will attempt ☝🏻 in another PR though because this one is large enough

[GeorgeMac]

👍 That is definitely PHP

[markphelps]

👍 That is definitely PHP

It do be