feat(core): Ensure startSpan() can handle spans that require parent
          #10386
        
          
      
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This introduces a new
onlyIfParentoption to the start-span APIs, which, if set, will ensure to not create a span if there is no parent. I've added this also for otel to demonstrate this works there as well.This PR also uses this option in a few places, esp. for http.client spans, to ensure we do not capture these if there is no ongoing transaction.
This replaces https://github.com/getsentry/sentry-javascript/pull/10375/files with a more generic solution, and includes the test from #10376 (which passes here as well).