Add a temporary seeded scenario generator receiver

[zzylol]

Part 2 of the RFC 0001 stack.

The official OpenTelemetry traffic generator cannot yet shape attribute cardinality, key popularity or value distributions, or change them over time: synthetic mode uses fixed value pools, and semantic_conventions mode sends every metric value as 1.0 (its TODO: "add configurable value range, distribution"). These options are planned upstream. Until then, this receiver stands in, with a config shaped like the upstream proposal so it can be swapped out.

What it does

urn:asap:receiver:scenario_generator sends Gauge points at signals_per_second, in max_batch_size batches, following the official generator's pacing and control handling.

config:
  seed: 1
  signals_per_second: 20000
  metric_name: http.server.request.duration
  attributes:
    - {key: http.route, cardinality: 200, zipf: 1.1}
    - {key: user.id, unique: true}
    - {key: k8s.pod.name, cardinality: 50, growth_per_minute: 0.05}
  value: {lognormal: {median: 50.0, sigma: 0.6}}
  changes:
    - {after_seconds: 600, attribute: http.route, equals: http.route-3,
       value: {lognormal: {median: 150.0, sigma: 0.6}}}
    - {after_seconds: 600, attribute: http.route, inject: /new/hot-route, share: 0.15}

Values are seeded, so a seed and config reproduce the same stream. Unique values include the seed, so two sources never share them.

Validation

Unit tests cover cardinality (fixed and unique), Zipf skew, changes after their time, and reproducibility. cargo fmt --check, cargo clippy --all-targets --features otap-engine,stream-benchmark -- -D warnings and cargo test --features otap-engine,stream-benchmark pass at this PR's head.