ddtrace/tracer: add support for span links on Span Context

[mhlidd]

What does this PR do?

Sets the framework that allows span links to be stored in extracted span contexts, and creates a new SpanContextWithLinks interface that enables access to such span links.

Motivation

In #2948, we update the functionality of chainedPropagator extract to create span links when there are conflicting trace ids in distributed tracing, and add those span links to the subsequently created span. In order to verify the proper functionality of this feature, this PR sets the framework of the updates in #2948 to allow us to write comprehensive system-tests.

APMAPI-883

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-08 21:25:13

Comparing candidate commit 4d35314 in PR branch mhlidd/prep_extracted_spanlinks with baseline commit 716a72c in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 58 metrics, 0 unstable metrics.

scenario:BenchmarkSetTagStringer-24

• 🟩 execution_time [-6.144ns; -3.936ns] or [-4.215%; -2.700%]