Add support for configuring proxy for communication with APM backend

At the moment Elastic APM PHP Agent does not have direct way to configure a proxy for communication with APM Server.
It can be configured indirectly using environment variables described at https://curl.se/libcurl/c/CURLOPT_PROXY.html#Environment
The cons of this approach are:
- Setting those environment variables might affect application code since it's running in the context of the same process
- It exposes agent implementation details (the fact that it uses curl for communication with APM backend which might change in the future)

Use links:
- Similar request for .NET agent - https://github.com/elastic/apm-agent-dotnet/issues/782