Allow setting the active span on the scope

[mattjohnsonpint]

With this change, the GetSpan method on the Scope class has been obsoleted, and replaced with a Span property that has both a getter and a setter.

Previous usage:

SentrySdk.ConfigureScope(scope =>
{
    var span = scope.GetSpan();
});

New usage:

SentrySdk.ConfigureScope(scope =>
{
    var span = scope.Span;
});

That's the boring part. The new interesting part is that you can now set the span if desired. This can be used to pin an active span as the root span before starting complex tasks such as doing work in parallel. This allows each subsequent child span to attach to the root, rather than to each other.

For example:

var span = SentrySdk.GetSpan();
Parallel.ForEach(items, item =>
{
    SentrySdk.WithScope(scope =>
    {
        scope.Span = span;

        // Do some work that might generate new child spans, etc.
        // They'll all be created from the span set on the scope.
    });
});

Or asynchronously:

var span = SentrySdk.GetSpan();
await Parallel.ForEachAsync(items, async (item, cancellationToken) =>
{
    await SentrySdk.WithScopeAsync(async scope =>
    {
        scope.Span = span;

        // Do some async work that might generate new child spans, etc.
        // They'll all be created from the span set on the scope.
    });
});

Or asynchronously with tasks:

var span = SentrySdk.GetSpan();
await Task.WhenAll(items.Select(async item =>
{
    await SentrySdk.WithScopeAsync(async scope =>
    {
        scope.Span = span;

        // Do some async work that might generate new child spans, etc.
        // They'll all be created from the span set on the scope.
    });
}));

Resolves #1845
Resolves #1679

[mattjohnsonpint]

Using the example from #1679, without this feature the spans would sometimes link to each other (because more than one parallel request is active at a time).

With this feature, the request spans are all rooted correctly: