[sdk-trace] make `activeSpanProcessor` private

[pichlermarc]

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Description

activeSpanProcessor is currently public and that allows users to modify the span processor by setting it on the fly even though the should be done via addSpanProcesssor() or - ideally - only via the constructor as the user is not supposed to hold on to the SDK instance of the TracerProvider after the initialization phase, as this can have unintended side-effects on the exported telemetry and should therefore be avoided.

This issue is considered done when

• [x] the activeSpanProcessor property at https://github.com/open-telemetry/opentelemetry-js/blob/47444f2a4c13a80f3328ffafc5b4c8db9e9a0886/packages/opentelemetry-sdk-trace-base/src/BasicTracerProvider.ts#L73 has been marked private or protected
• [ ] it has been verified that activeSpanProcessor is not public in any other way for the BasicTracerProvider
• [ ] it has been verified that activeSpanProcessor is not public in any other way for the WebTracerProvider
• [ ] it has been verified that activeSpanProcessor is not public in any other way for the NodeTracerProvider