phpstan-ddtrace extension

The phpstan STUBS for the datadog/dd-trace-php, but is more convenient.

Do you use the DataDog Tracing library for PHP? Do you also care about the quality of your code? Great. Now, you can use dd-trace-php stubs without installing the whole dd-trace composer library that has everything that is not interesting to you because it's still better to use dd-trace as a php extension.

1. Install the dd-trace extension

// TODO

2. Install this phpstan extension

// TODO

3. Add the extension to the phpstan configuration

4. Run phpstan