[Instrumentation.AWS]: always add context propagation data to requests

[cfbao]

Fixes #2345

Changes

Move code that adds context propagation data to requests into a dedicated method AddPropagationDataToRequest, and always run that regardless of sampling decision.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

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• ✅ login: cfbao / name: Chenfeng Bao (c922146, ec7a0d8)

[cfbao]

There's no existing unit tests in this area, but I've verified that correctly adds relevant message attribute to SQS requests using the script showed in #2345.