Capture Activity trace and span ids when present

[nblumhardt]

By default, the built-in trace and span ids will be recorded alongside any span-related properties (such as TraceId, SpanId, and ParentId) that may be added by the framework:

To remove the redundant properties (if you wish) you can set ActivityTrackingOptions via ILoggingBuilder.Configure():

builder.Logging.Configure(opts => opts.ActivityTrackingOptions = ActivityTrackingOptions.None);

The PR draws on the trace and span correlation support I added upstream in Serilog, and I've followed it pretty closely here.