Allow circuit-specific Circomkit config overrides

[erhant]

For example, we may set plonk as the Circomkit protocol in circomkit.json, but we would also like something such as:

{
  "multiplier_3": {
    "file": "multiplier",
    "template": "Multiplier",
    "params": [3],
    "overrides:" {
        // circomkit overrides here!
        "version": "2.1.6",
        "protocol": "groth16"
    }
  },
}