statement-store: implement new rpc api

[alexggh]

Work in progress

Description

Implements the proposed changes from here #10452, where the API surface has been limited to two functions submit and a subscribe_statement.

Submit changes

For submitting the only changes are implementing the logic of the newly expiry field where statements with an expiration timestamp lower than current time get rejected.

Subscribe changes

For subscription the approach is to have a configurable number of workers that keep track of all subscriptions, each time a new subscription arrives it gets assigned by a round-robin protocol to one of the workers.

When a new statements gets accepted by the statement-store it notifies all workers about the statement, the workers then go and evaluate all subscription filters they are assigned to and notifies each of them accordingly.

Remaining work

• Add more tests.
• Evaluate performance under load and port statement_store_bench.rs to using subscribe API, ported and performance stay the same on the benchmark, because bottlenecks are elsewhere and being addressed.

[2026-01-07T17:18:51Z INFO  tests::zombie_ci::statement_store_bench] Participants: 49998, each sent: 6, received: 0
[2026-01-07T17:18:51Z INFO  tests::zombie_ci::statement_store_bench] Summary        min       avg       max
[2026-01-07T17:18:51Z INFO  tests::zombie_ci::statement_store_bench]  time, s         8        19        24
[2026-01-07T17:18:51Z INFO  tests::zombie_ci::statement_store_bench]  retries         0         0         0

• Evaluate and set proper RPC limits. The normal RPC limits apply here we just need to make sure RPC nodes run with limits high enough, so each user can connect to the node all at the same time.
• Periodically scan existing statements and removed expired ones.
• Plan for deploying it with minimising impact for people prototype with the old APIs
  - [ ] This modifies statement format(expiry field), that means chain runtime would have to be update to use the new format.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/20858252731
Failed job name: test-linux-stable-int