[8.18] (backport #9218) Refactor binary downloads in agent packaging targets

[mergify]

What does this PR do?

Refactors how the agent packaging target downloads binaries. The intent is to make it possible to cache the binaries locally and avoid having to redownload them. See #9133 for the full context.

The PR consists of the following changes:

• The download function now always gets a target path as an input, so it doesn't need to create it on its own conditionally.
• We use standard library functions for the download, so we don't need to pass the content as a string, and can instead write it directly to the target file, saving memory.
• Remove some unused functions.

Why is it important?

This makes the download code more maintainable and more performant. It also unlocks further packaging improvements.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

How to test this PR locally

Related issues

• Relates https://github.com/elastic/observability-dev/issues/4702

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This is an automatic backport of pull request #9218 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

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• Commit: 851edb6

cc @swiatekm