Use base64+json for trace context encoding

[goodspark]

Back when I added trace propagation to the agent, I was short-sighted and used the golang-binary format. Since the trace context was meant to be propagated between builds of a pipeline, it was safe to assume the consumers would always be able to decode golang-binary formats.

But now I want to trace additional spans within non-Golang code and link it to the parent trace of the pipeline build. This will let me gather hollistic metrics about specific failure modes and performance of various commands/tooling running in my repos.

So I picked JSON, since it's a format all modern languages can easily parse without the need for a special library.

TODOs

• [x] Tests
• [x] Test a build and verify traces go through and trace context is properly passed to subprocesses

[agates4]

I'm excited about this PR - it would help us send additional traces through to Datadog!

[goodspark]

This has been sitting around for months. Any chance someone from Buildkite can review this?

[smocherla-brex]

Hey folks, is there anything we could do to merge this PR? This would be really useful to add custom trace spans in our commands running in CI.