Fluent methods

[stayallive]

Implement fluent methods on Options and tracing related classes.

I've also updated a few tests just to give some examples.

// From:
$spanContext1 = new SpanContext();
$spanContext1->setSampled(false);
$spanContext1->setSpanId(SpanId::generate());
$spanContext1->setTraceId(TraceId::generate());

// To:
$spanContext1 = (new SpanContext())
    ->setSampled(false)
    ->setSpanId(SpanId::generate())
    ->setTraceId(TraceId::generate());

It's not less code but it does write nicer and as far as I can tell there is no downside to doing this so shouldn't be much against doing this by default.