Background agent and flush interval support

[marandaneto]

Description

Since PHP is singlethreaded, we cannot do the same as Python, Go, etc

An idea would be:

• Add lib/Consumer/Agent.php
• Add "agent" => Agent::class to Client::CONSUMERS
• Add config:
  • agent_host
  • agent_port
  • agent_timeout_ms
  • agent_fallback
• Add a separate executable, maybe bin/posthog-agent, or document using a PostHog sidecar/daemon if one exists.
• Agent batches events and posts to /batch/.