feat(core): implement Tracing without Performance

[lcian]

Implements Tracing without Performance (spec).

This is done by having an additional PropagationContext attribute on the scope that holds a "fake" trace_id and span_id.
When applying the scope to an event, if there is no ongoing span set on the scope, we create the trace context from the PropagationContext.
Additionally, adds an API scope.iter_trace_propagation_headers that should be preferred over something like:

sentry::configure_scope(|scope| {
	if let Some(span) = scope.get_span() {
		headers = Some(span.iter_headers());
	} 
});

because it will use the PropagationContext as a fallback.

Closes #796

Also makes a minor adjustment to trace and span ids where we now use the Display impl for Debug, otherwise we were getting u8 arrays when using e.g. dbg! with the derived one which is not very useful.

[codecov]

Codecov Report

Attention: Patch coverage is 58.62069% with 24 lines in your changes missing coverage. Please review.

Project coverage is 72.99%. Comparing base (bec93d2) to head (8cf93b8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   73.23%   72.99%   -0.25%     
==========================================
  Files          62       62              
  Lines        7238     7279      +41     
==========================================
+ Hits         5301     5313      +12     
- Misses       1937     1966      +29     

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[szokeasaurusrex]

Looks good overall, I have some (mostly minor) suggestions though

[lcian]

The idea is that this could also return the ids from PropagationContext if there is no ongoing span, not sure if this comment correctly reflects that.
But on the other hand we should also hide PropagationContext as that's an "implementation detail".

[lcian]

If client is not active then we will use the noop scope that doesn't call this