Move mock proxy test server from elastic-agent to elastic-agent-libs for common use

[khushijain21]

What does this PR do?

This PR moves mock proxy server files from https://github.com/elastic/elastic-agent/blob/615190be3f6a28403883f6b4983764558173e0e7/testing/proxytest/proxytest.go

and https://github.com/elastic/elastic-agent/blob/615190be3f6a28403883f6b4983764558173e0e7/testing/proxytest/https.go

to elastic-agent-libs so that they can be used by beats as well.

It also makes a small change to the original code. proxy-server now writes the incoming headers first and then the status code.

Why is it important?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Required for [beats receivers] Test that translated Elasticsearch exporter configurations work with: self-signed certificates, mTLS, and proxies beats#45491

[AndersonQ]

@khushijain21, could you bring the whole proxytest package from the agent repo? On elastic-agent/testing/proxytest there are 2 important files which are not there, the tests for the proxy and an example of how to use it as a "cli" application.

[belimawr]

I agree with @AndersonQ, it's better to bring the whole package, which includes the tests.

It also makes a small change to the original code. proxy-server now writes the incoming headers first and then the status code.

This is actually a bugfix. The headers should be fully set before calling WriteHeader. Thanks for fixing that!

[khushijain21]

could you bring the whole proxytest package from the agent repo?

done

[elasticmachine]

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