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 72.81%. Comparing base (ae9779f) to head (10d27f9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   73.05%   72.81%   -0.25%     
==========================================
  Files          63       63              
  Lines        7267     7308      +41     
==========================================
+ Hits         5309     5321      +12     
- Misses       1958     1987      +29     

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[szokeasaurusrex]

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

[szokeasaurusrex]

Looks good, please address comments before merging (I marked some as nit or optional, you can ignore those if you want)

[github-actions]

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