feat(sinks): add network_mock sink

[mdeltito]

This sink functions like a blackhole sink, but exercises the full lifecycle of other sinks that make network requests using a Tower service (batching, encoding, request building, concurrency control, retries, acknowledgements). Supports configurable latency (mean + jitter), error rate simulation, and standard batch/request settings.

Ref: LOG-23599

Example

[api]
enabled = true

[sources.demo]
type = "demo_logs"
format = "json"
interval = 0.01

[sinks.mock]
type = "network_mock"
inputs = ["demo"]
error_rate = 0.5

  [sinks.mock.latency]
  mean_ms = 50
  jitter_ms = 10

  [sinks.mock.buffer]
  max_events = ${MEZMO_MEM_BUFFER_MAX_EVENTS:-10000}
  when_full = "block"

  [sinks.mock.batch]
  max_bytes = 2_097_151

  [sinks.mock.request]
  retry_attempts = 10
  retry_initial_backoff_secs = 1
  retry_max_duration_secs = 10
  timeout_secs = 30
  rate_limit_duration_secs = 1

  [sinks.mock.acknowledegments]
  enabled = true