[libbeat] aws auth method: fix service name and region overwrite

[moukoublen]

Proposed commit message

Fix the service name and region overwrite when values are set in the input configuration in the aws auth method that was introduced here.

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

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[github-actions]

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @moukoublen? 🙏.
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-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[khushijain21]

LGTM

[olegsu]

@Mergifyio backport 8.19

[mergify]

backport 8.19

✅ Backports have been created

Details

• #50155 [8.19](backport #47712) [libbeat] aws auth method: fix service name and region overwrite has been created for branch 8.19 but encountered conflicts

Cherry-pick of 183016c has failed:

On branch mergify/bp/8.19/pr-47712
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 183016ce9.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   x-pack/libbeat/common/aws/signer.go
	modified:   x-pack/libbeat/common/aws/signer_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/libbeat/common/aws/credentials_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally