feat: expose TraceId and ParentID to other plugins

[dmehala]

Description

This commit adds two fields to kong.ctx.shared and ngx.var:

• datadog_sdk_trace_id: TraceId of the current span.
• datadog_sdk_span_id: SpanId of the current span.

Those fields can be consumed by other Kong plugins or in the underlying NGINX configuration for log correlation.

Resolves #25

[dmehala]

Hey @pablomartinezbernardo

Since your last review, I added an example. Would you mind checking if it works for you? 🙇

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.68%. Comparing base (15d7bd3) to head (ac21681).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #61 +/- ## ======================================= Coverage 92.68% 92.68% ======================================= Files 7 7 Lines 615 615 ======================================= Hits 570 570 Misses 45 45 ```

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