feat: flexible canister http outcalls

[mraszyk]

Summary

• New flexible_http_request management canister method: a variant of http_request where nodes return their individual HTTP responses to the caller instead of reaching consensus.
  Supports configurable replication via an optional replication argument (min_responses, max_responses, total_requests). Returns a vector of responses or a structured error with a global error code and per-node resource reports.
• New pricing_version field on http_request: adds an optional pricing_version field (1 or 2). Version 1 is now deprecated; version 2 ignores max_response_bytes and refunds unused cycles based on actual execution cost.
• New ic0.cost_http_request_v2 System API: replaces the deprecated ic0.cost_http_request. Accepts a Candid-encoded parameter record to compute the cost of an outcall at pricing version 2.
• New ic0.subnet_self_node_count System API: returns the number of nodes currently on the subnet. Useful for callers computing valid replication bounds for flexible_http_request.

[github-actions]

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