Improve docs regarding https outcalls to mention *is_replicated* flag

[3erikpotter]

Current technical guides for http outcalls mention the topic of query idempotence. That for non-idempotent POST queries, the default behaviour of ICP consensus fires many requests, gets different results back and hence fails the consensus. We can't avoid this with transformer functions. To counter this, there are proxy services mentioned that were created by community.
https://juno.build/docs/guides/rust#technical-requirements

But since 2025 August, there is somewhat experimental feature launched within the protocol to use is_replicated flag. This is likely a prefered and easiest way for many use cases where not having replication is a necessary compromise of bridging with existing traditional systems.
https://forum.dfinity.org/t/announcing-two-major-upgrades-for-https-outcalls-ipv4-support-non-replicated-calls-are-now-live/54580

This is enabled since ic-cdk version 0.19.0