feat(php): Implement dynamic snippets

[amckinney]

This implements the dynamic snippets for PHP. A couple of core features (e.g. discriminated unions) still need to be finished to resolve all of the cases, so a few cases are added to the allowedFailures set seed.yml.

Along the way, this also caught a couple of issues that are also resolved here:

1. We don't require the user to construct an inlined request object if it's composed of entirely optional fields.
2. We map a list of optional query parameters into an optional list of query parameters.
   • This is far more convenient for the user and was the intended use case.
   • For example, array<?string> -> ?array<string>

This also includes the integration with seed so that every snapshot will have a separate dynamic-snippets directory containing a variety of snippet examples. This ensures the snippet generator is valid for each test definition.