Avoid SpanBuilder allocation for startSpan (#9998)

* Extracting static helper

* Extract static version of buildSpanContext

* Switch to singleSpanBuilder

* Extracting static version of startSpan

* spotless

* Reorganizing link & parent resolution

Previous static extraction caused a test to fail because links logic relied on having a side effect on the builder instance's List<AgentSpanLink>

* spotless

* Apply suggestion from @mcculls

Co-authored-by: Stuart McCulloch <stuart.mcculloch@datadoghq.com>

* Addressing review feedback - Change CoreSpanBuilder constants to be more intelligible

* Addressing review feedback - using constants throughout

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Co-authored-by: Stuart McCulloch <stuart.mcculloch@datadoghq.com>

Author: dougqh