feat(benchmark): add CompetingConsumerThroughputBenchmark harness (#718)

[Shubh2-0]

Relates to #718:

• Goal: Introduce a JMH benchmark harness (\CompetingConsumerThroughputBenchmark) under the \�enchmark\ module to measure baseline throughput of \CompetingConsumerSubscriptionModel\ across varying consumer counts (1, 2, 4, 8) as requested in Support parallel competing consumers without a single subscription-wide lease #718.
• Scope: Includes \CompetingConsumerSubscriptionModel\ dependencies in \�enchmark/pom.xml\ and standard JMH \Throughput\ configuration.