Add support for configuring proxy for communication with APM backend

[SergeyKleyman]

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 - Proxy support apm-agent-dotnet#782