Move span pointer to inferred span; add env var to toggle span pointers

[nhulston]

What does this PR do?

1. Adds an env var DD_TRACE_AWS_ADD_SPAN_POINTERS to toggle span pointers
2. Put span pointers on inferred (root) span instead of the lambda (child) span, as discussed in Slack.
3. Wrap span.addSpanPointers with try/catch, since span is of type any.

Note that moving the location of the span pointer has no actual effect on the functionality and requires no changes on the UI side. As long as a span is found with a matching hash, span pointers work. Since the spans are on the same trace, this has no noticeable change for our users. This change just aligns span pointers with bottlecap (universally instrumented) runtimes.

Motivation

Testing Guidelines

Manual testing. The span links are now on the root (inferred) span.

Additional Notes

Types of Changes

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

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• This PR contains breaking changes that are documented in the description
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