Add ISpan.GetTransaction convenience method

[mattjohnsonpint]

This adds a convenience method for when you have an ISpan and need to get the ITransaction it belongs to.

For example:

void DoSomething(ISpan span)
{
    var transaction = span.GetTransaction();
}

This also works well anywhere we have a GetSpan method, such as IHub and SentrySdk

var hub = serviceProvider.GetRequiredService<IHub>();
var transaction = hub.GetSpan()?.GetTransaction();

or

var transaction = SentrySdk.GetSpan()?.GetTransaction();

Motivation

With custom measurements added in #2013, we have this:

transaction.SetMeasurement("some_time", 50, MeasurementUnit.Duration.Millisecond);

Since sometimes we only have a span, we need to do this:

span.GetTransaction().SetMeasurement("some_time", 50, MeasurementUnit.Duration.Millisecond);

The alternative would have been to attach SetMeasurement extension methods on ISpan as well, but that would imply that measurement data is set on individual spans - which it is not.

This seems cleaner and has other usages as well.

[mattjohnsonpint]

Some minor formatting changes in the PR as well.