Allow Custom CA Certificates

**Describe the bug**
Users who have custom certificate authorities in the chain are not able to use TLS validation, as there is no way to add more trusted CA certificates.

The Node agent has the option [serverCaCertFile](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#server-ca-cert-file), but the PHP agent does not have an equivalent option.

Disabling TLS verification is not an acceptable solution.

**To Reproduce**
Steps to reproduce the behavior:
1. Put the APM server behind a custom CA
2. Try to use the PHP Elastic APM Agent
3. Notice cert errors in the PHP logs
4. Try to find the option for custom CA certs
5. Fail to find such an option

**Expected behavior**
The agent should be usable with custom CAs in the chain without circumventing security.