feat(php): Implement RawClient.php

[amckinney]

This adds a variety of the Core package utilities, all of which are required for the RawClient type, which is the primary mechanism used to make HTTP requests.

Several things to note:

1. The BaseApiRequest will be used by every flavor of HTTP endpoint call (i.e. JSON, multipart form, streaming, etc).
   • For now, we only implement JSON.
2. The RawClientTest.php file is a set of simple tests to confirm the RawClient acts as expected.
   • We can add more tests over time, but this at least sets the stage.
3. The RawClient currently uses the synchronous method call.
   • This will either be refactored to be exclusively async, or an additional method will be rolled out to support those use cases.
   • It's unclear if we will need to support both options, or simply just async.
4. Tidies up a few things w.r.t. namespace from the original PhpProject PR here.