fix(iast): avoid native module import if iast disabled [backport 2.6]

[github-actions]

Backport b4a1d97 from #8564 to 2.6.

IAST: Forces an ImportError if the IAST _taint_tracking native module is imported when IAST is not enabled, to ensure there are no side_effects, like segmentation faults, if IAST is not enabled.

Fixes #8504

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.
• If change touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-8564-to-2.6
Commit report: 5286c39
Test service: dd-trace-py

✅ 0 Failed, 339 Passed, 110784 Skipped, 16m 50.93s Total duration (1h 39m 2.61s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-03-05 19:15:41

Comparing candidate commit 88f3830 in PR branch backport-8564-to-2.6 with baseline commit cc5c42e in branch 2.6.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 192 metrics, 9 unstable metrics.

scenario:otelspan-start

• 🟥 max_rss_usage [+3.494MB; +3.642MB] or [+7.317%; +7.628%]