[NET-1638] implement findProxyNodes in sdk

[hpihkala]

Summary

See below bugbot summary.

Checklist before requesting a review

• Is this a breaking change? If it is, be clear in summary.
• Read through code myself one more time.
• Make sure any and all TODO comments left behind are meant to be left in.
• Has reasonable passing test coverage?
• Updated changelog if applicable.
• Updated documentation if applicable.

---

Note

Introduce StreamrClient#findProxyNodes using a new ProxyNodeFinder, update OperatorRegistry#findOperatorsOnStream to return operator IDs with peer descriptors, add tests, and remove internal findOperators.

• SDK (@streamr/sdk)
  • New:
    • ProxyNodeFinder: discovers proxy nodes from operators with parallelized search.
    • StreamrClient#findProxyNodes(streamDefinition, numberOfProxies, maxQueryResults, maxHeartbeatAgeHours) delegates to ProxyNodeFinder.
  • OperatorRegistry API:
    • findOperatorsOnStream() now returns { operatorId, peerDescriptor } instead of peer descriptors only.
  • Removed:
    • Internal StreamrClient#findOperators.
  • Tests:
    • Add ProxyNodeFinder unit tests and update FakeOperatorRegistry to new return type.
  • Docs:
    • Update CHANGELOG.md reflecting additions and removal.

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[linear]

NET-1638 Implement findProxyNodes in SDK

[juslesan]

Looks OK, a couple of minor comments.

As a heads up we might not support "bidirectional" proxies, meaning that proxies are only able to publish or subscribe to messages on the network level. For the StreamrTV use case we might need to add support for bidirectionality as the viewers need to be able to listen to messages and to send out heartbeats

[hpihkala]

As a heads up we might not support "bidirectional" proxies

Ah, damn, that complicates things! We will definitely need bidirectionality. I created a ticket and assigned it to you :)