[SLES-1463] [bugfix] ddtrace spancontext traceid need to be object

[joeyzhao2018]

What does this PR do?

This PR ensures that the traceId and spanId are boxed as a String object when creating the dd-trace/span-context type (so that later it won't error out when calling toArray())

Motivation

We have a customer issue linked here. The error stacktrace is

2023-12-07T13:57:10.020Z    c241f8c3-8001-4a5d-a333-6ae15155d2a2    DEBUG   {
    "status": "debug",
    "message": "id.toArray is not a function",
    "name": "TypeError",
    "stack": "TypeError: id.toArray is not a function\n    at AgentEncoder._encodeId (/var/task/node_modules/dd-trace/packages/dd-trace/src/encode/0.4.js:168:13)\n    at AgentEncoder._encode (/var/task/node_modules/dd-trace/packages/dd-trace/src/encode/0.4.js:96:12)\n    at AgentEncoder.encode (/var/task/node_modules/dd-trace/packages/dd-trace/src/encode/0.4.js:45:10)\n    at Writer._encode (/var/task/node_modules/dd-trace/packages/dd-trace/src/exporters/common/writer.js:39:19)\n    at Writer.append (/var/task/node_modules/dd-trace/packages/dd-trace/src/exporters/common/writer.js:35:10)\n    at AgentExporter.export (/var/task/node_modules/dd-trace/packages/dd-trace/src/exporters/agent/index.js:45:18)\n    at SpanProcessor.process (/var/task/node_modules/dd-trace/packages/dd-trace/src/span_processor.js:53:30)\n    at DatadogSpan.finish (/var/task/node_modules/dd-trace/packages/dd-trace/src/opentracing/span.js:183:21)\n    at SpanWrapper.finish (/var/task/node_modules/datadog-lambda-js/dist/trace/span-wrapper.js:28:19)\n    at TraceListener.<anonymous> (/var/task/node_modules/datadog-lambda-js/dist/trace/listener.js:227:39)"
}

This issue can be replicated when trying to passing down x-ray trace context with DD_MERGE_XRAY_TRACES: true

Testing Guidelines

• added one unittest to ensure the type
• tested with reproduced example in sandbox account

Additional Notes

Types of Changes

• [x] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [x] This PR passes the integration tests (ask a Datadog member to run the tests)