feat(outcomes): Add no_parent_span outcome

[Lms24]

This PR adds a new outcome for when spans are not started (or dropped) due to no parent span being present at span start.
This is a common pattern across SDKs. Initial use case: suppressed http.client spans for requests happening outside of active spans.

ref https://linear.app/getsentry/issue/SDK-1123/add-client-report-outcome-for-dropped-spans-because-of-missing-parent

[linear-code]

SDK-1123 Add client report outcome for dropped spans because of missing parent spans

[cursor]

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