`/nipopow/proof` REST API endpoint support

[greenhat]

Implement ergo_rest::api::node::get_nipopow_proof_by_header_id using /nipopow/proof/{m}/{k}/{headerId} REST API endpoint.

Add it to the Wasm and C/Swift API. See existing node::get_info code for examples.

[kettlebell]

Will be done as part of #544.