[13.x] Allow Http Client to be used as PSR Client

[willrowe]

This makes some options used internally by the Http client optional so that the client returned by buildClient can be used anywhere that an instance of \Psr\Http\Client\ClientInterface is required. This allows the middleware and the handler stack to be used on all requests made using the client, which means things like request logging will also apply.

For example, say your application uses a package that allows address geocoding and has different drivers available for different map providers. The service class in this package must be instantiated with an instance of \Psr\Http\Client\ClientInterface, which it then uses to make API requests to the corresponding map providers. By passing the Laravel Http client, all requests and responses from these APIs can then be logged alongside all the other Http requests being made directly by application code.

$geocoder = new GeocodingService(Http::buildClient());