Migrate the PHP client generator to elasticsearch-specification

The PHP client generator currently uses rest-api-spec as a source. It should be adapted to use [elasticsearch-specification](https://github.com/elastic/elasticsearch-specification) instead, like all the other clients do.

These are the uses of rest-api-spec that need to be addressed:

- [ ] The main generator loop, which iterates over the endpoints
- [x] The endpoint availability, which determines which endpoints are generated and which are skipped
- [ ] The comment block with the arguments, included above each endpoint function
- [ ] The default values of arguments
- [ ] Which inputs (path parts, params, body) are required
- [ ] Which headers to send with each endpoint