Datadog plugin does not support DD_TRACE_AGENT_URL (Unix socket) while official Datadog agent does

### Description Hi team We’ve noticed that the Kong Datadog plugin currently does not consider Unix sockets as a valid agent host, while the official Datadog agent, libraries, and SDKs support configuring the trace agent via the `DD_TRACE_AGENT_URL` environment variable. This creates a gap when running Datadog in environments where Unix domain sockets are preferred or required (e.g., hardened environments, containerized deployments, or when avoiding TCP exposure). ### Current behavior - The Datadog plugin expects a host/port configuration - Unix sockets are rejected or ignored - `DD_TRACE_AGENT_URL` is not honored, even when set ### Expected behavior - Support `DD_TRACE_AGENT_URL` as defined by Datadog - Allow values such as: `unix:///var/run/datadog/apm.socket` - When `DD_TRACE_AGENT_URL` is set, it should override host/port configuration, consistent with Datadog’s official behavior ### Why this matters Datadog officially documents and supports `DD_TRACE_AGENT_URL` across: - Datadog Agent - Tracing libraries and SDKs Using Unix sockets is: - More secure (no exposed TCP port) - Common in Kubernetes and container-based setups - Already supported by Datadog tooling, but blocked by the Kong plugin Aligning Kong’s Datadog integration with Datadog’s standard configuration would improve compatibility and reduce operational friction.