chore: add arg k_top_api_nodes for dynamic routing to best k nodes

[nikolay-komarevskiy]

For example setting ic_use_k_top_api_nodes=4 would cause routing to only 4 API nodes using weighted random sampling. NOTE: this is different from the n_ordered_routes().

In /metrics this would manifest in:

# HELP ic_gateway_api_boundary_nodes Number of API boundary nodes with status.
# TYPE ic_gateway_api_boundary_nodes gauge
ic_gateway_api_boundary_nodes{status="healthy"} 4
ic_gateway_api_boundary_nodes{status="total"} 20

We might need another status in_routing as healthy is a bit misleading here.