Add shared client report aggregator and send_client_reports option

[szokeasaurusrex]

Part of #882. Depends on #1001.

Implement a shared, thread-safe client report aggregation primitive (for example keyed by (reason, category) with record and drain APIs) and add ClientOptions::send_client_reports defaulting to true. Wire the option through client initialization so bookkeeping and report emission can be fully disabled when requested.

[linear]

RUST-154