server, bark: use dedicated GetOffboardFeeRate RPC instead of caching ark_info

Kobe Nysrequested to merge
GetOffboardFeeRate-grpc into master

The offboard fee rate was the only dynamic field in ArkInfo, requiring TTL-based caching and periodic re-fetches of the entire ArkInfo response. Add a dedicated GetOffboardFeeRate endpoint so bark can fetch the current rate directly when needed. The field in ArkInfo is marked deprecated but still populated for older clients.

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