Cleanup: Replace custom goroutine shutdown listeners with shared context wrapper

[faec]

Simplify context handling in the awscloudwatch and awss3 inputs.

The awscloudwatch and awss3 inputs both need to convert their Beats input context to a standard golang context for use with the AWS API, and both of them use custom wrappers that create an extra goroutine to listen for Beat shutdown. I added an explicit wrapper object to the v2.InputContext API that is a single function call and requires no extra goroutines, and replaced the context wrapper in both inputs.

This doesn't change any functional behavior.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @faec? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[elasticmachine]

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[zmoog]

v2.GoContextFromCanceler() is a handy addition to input v2.