Make SpanProcessor::on_start take a mutable span

[ramosbugs]

As a side effect of #505, SpanProcessor::on_start callbacks are no
longer able to modify the span. The OpenTelemetry spec states that the
span parameter is a read/write span object. This PR introduces a breaking
change to restore adherence to the spec.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@197c388). Click here to learn what that means.
The diff coverage is 60.0%.

@@          Coverage Diff           @@
##             main    #601   +/-   ##
======================================
  Coverage        ?   55.0%           
======================================
  Files           ?     100           
  Lines           ?    8890           
  Branches        ?       0           
======================================
  Hits            ?    4895           
  Misses          ?    3995           
  Partials        ?       0           

Impacted Files	Coverage Δ
opentelemetry/src/sdk/trace/provider.rs	81.8% <0.0%> (ø)
opentelemetry/src/sdk/trace/span_processor.rs	81.7% <50.0%> (ø)
opentelemetry/src/sdk/trace/tracer.rs	88.4% <100.0%> (ø)

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[TommyCpp]

👍 Looks good! Thanks. This API may need further changes to allow implementation to keep a reference to the span. As stated in spec,

It SHOULD be possible to keep a reference to this span object and updates to the span SHOULD be reflected in it

[jtescher]

@ramosbugs looks great thanks, @TommyCpp yeah looks like the spec wants that, I think it's fine for now as that's just a SHOULD at this point.

[ramosbugs]

thank you both for the quick merge!