Adds support for generating 128 bit trace ids and exposing them from the Datadog API via a new SpanContextW3C. Future PRs will propagate this id for distributed tracing.

Remove the WithTraceID128 API that was added to this feature branch (not currently in a dd-trace-go release or in main).

Support DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED to only log 128 bit trace id values when enabled.

Rather than generate a random 64 bits for the upper half of a 128-bit trace id, use the first 32 bits as a timestamp, and the remaining 32 bits padded with zeroes.

Supports 128-bit trace id propagation in B3 multi headers. Other carriers will be supported in future PRs, and TODOs have been added in the code where it is needed.

Also fixes the issue where a spanContext needed to have an associated span to access and store 128-bit trace ids. It is now a separate field of the spanContext. Now, the _dd.p.tid tag in the Meta map (upper 64 bits of 128-bit trace ID) is populated from the spanContext upon span finish.

Small refactor to tests to make it easier to test with 128-bit trace ids for other carriers.

These tests could probably be refactored even further to have less repetition, but for now, it's just extending them to support 128-bit trace ids.

Supports 128-bit trace propagation of the remaining carriers per the specification.

When DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED is false, only the lower-order 64-bits of the TraceId should be logged.
When DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED is true, the format of the injected TraceId is conditional based on the higher-order 64 bits of the TraceId - specifically, when:
zero - the injected TraceId will be its decimal string encoding (preserving the current behavior of 64-bit TraceIds)
non-zero - the injected TraceId will be encoded as 32 lower-case hexadecimal characters with zero-padding as necessary

- guarantee that the propagating tags have been set before reading them during extraction
- only set _dd.p.tid if the top 64 bits is not empty or zero
- always set _dd.p.tid to 16 characters, with padding if needed

* WIP: more refactoring, still broken tests though
* Even more bug fixes
* Update ddtrace/tracer/spancontext.go
* PR Comments: use strconv.ParseUint instead of hex.Decode since it can handle odd length strings
Co-authored-by: Katie Hockman <katie@hockman.dev>