Allow extend SdkSpanBuilder

Is your feature request related to a problem? Please describe.
I am trying to make some special marker for root span distinguish with child span to double check trace data integrity. So I want add some logic during SpanBuilder.startSpan() 。

Sure, SpanBuidler is a interface, I can implement my own MySpanBuilder, But I just need some small tweaks, I do not want to Copy & Paste code from SdkSpanBuilder . SdkSpanBuilder also coupled with OpentelemetrySdk, AutoConfiguredOpenTelemetrySdk which used by opentelemetry-java-instrumentation project tightly. It's a big project to implement my own MySpanBuilder and integrate with opentelemetry-java-instrumentation .

Describe the solution you'd like

Add some SdkSpanBuilderCustomizer function interface called after SdkSpanBuilder.startSpan().

Describe alternatives you've considered

OpentelemetrySdkBuilder accept raw MeterProvider, TraceProvider interface type instance to allow add my own TraceProvider may be an alternative method to archive same goal.

Start with SdkSpanBuilder.startSpan() and the OpentelemetrySdkBuilder APIs mentioned in the issue. Compare the requested SdkSpanBuilderCustomizer approach with accepting raw provider interfaces, then define the public extension point and verify that it supports customization without copying SdkSpanBuilder or disrupting opentelemetry-java-instrumentation integration.