Describe the enhancement:

Extend the testing/proxytest to support mTLS. It should be possible to configure:

• the CA the proxy will use for its own requests to other servers
• the CA to validate the clients' TLS certificate
• the TLS certificate to present to its clients
• the key for the certificate presented to its clients

Describe a specific use case for the enhancement or feature:

Test the Elastic Agent support for mTLS between itself and fleet-server or a proxy. More specifically, to do an integration test for test

• Load fleet.ssl.certificate_authorities from agent policy #2247
• Load fleet.ssl.certificate and fleet.ssl.key from agent policy #2248

What is the definition of done?

cURL can use the testproxy when it requires mTLS. The request will only succeed if cURL is correctly configured to present TLS certificates and with the proxy's CA.

The proxy is configured with:

• cURL CA's
• proxy-cert.pem
• procy-cert-key.pem

cURL is used to reach https://elastic.co as follows:

curl --cacert proxy-ca.pem \
     --cert cRUL-cert.pem \
     --key cRUL-cert-key.pem \
     https://elastic.co