SLF4J sink for decision events

The existing opa-slf4j module routes the SDK's operational logger to SLF4J — it doesn't expose decision events as SLF4J log records. Without that, users have to either accept the JSON-array HTTP upload path or scrape the console: true plain-text output to integrate with their existing log pipeline (Logback, ELK, MDC, async appenders).

• Add an SLF4J sink for decision events (configurable, independent of the service uploader): one log record per event on a dedicated logger name (e.g. io.github.open_policy_agent.opa.decisions).
• Marker / structured-args support so users can route via Logback's JSON encoder or Logstash encoder.
• Configurable level (default INFO).
• Tests: assert events appear on the named logger and survive mask_decision / drop_decision (depends on #73 / #74).
• Document recommended Logback config snippets for JSON output.

Start by reading the existing opa-slf4j module and the decision-event paths behind the JSON-array upload and console: true output. Review #73 and #74 before defining tests for masking and dropping decisions. Done means a configurable dedicated logger emits one event per record with structured arguments or markers, supports a default INFO level, and includes recommended Logback JSON configuration documentation.