[8.19](backport #48956) Use combined External ID for AWS cloud connectors AssumeRole

[mergify]

Summary

This change updates the AWS cloud connectors credential flow in libbeat so that the AssumeRole step uses a combined External ID built from the cloud resource ID and the configured external ID: CloudConnectorsExternalID(resourceID, externalIDPart) returns resourceID-externalIDPart. This allows the remote (customer) role trust policy to scope access by resource while still using the configured external ID. The credential chain is unchanged: assume the Elastic global role with web identity (OIDC token), then assume the configured role with the new External ID and optional expiry window. Tests are updated to assert the new External ID format and a unit test is added for CloudConnectorsExternalID.

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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This is an automatic backport of pull request #48956 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f909d4f has failed:

On branch mergify/bp/8.19/pr-48956
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit f909d4fb7.
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Unmerged paths:
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	deleted by us:   x-pack/libbeat/common/aws/cloud_connectors.go
	deleted by us:   x-pack/libbeat/common/aws/cloud_connectors_test.go

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[github-actions]

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[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[olegsu]

More context: #49956