Mtoff/span events

[mtoffl01]

What does this PR do?

Adds support for span events using the Opentelemetry AddEvent API in dd-trace-go.

Motivation

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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-07-15 19:01:19

Comparing candidate commit 6c7ab6c in PR branch mtoff/span-events with baseline commit 974aea6 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 46 metrics, 1 unstable metrics.

[darccio]

Suggested change

	type SpanEvent struct {
	type spanEvent struct {

Do we need to export this? Could it be an internal type?

[darccio]

Approving as it's good to go, with only the minor edit of spanEvent pending to apply.