Add JSON export support

[nymkappa]

This PR adds a new json value to the timemachine restore --fmt command option, allowing the user to output the graph in JSON format directly.

This is useful for programming languages with native JSON support like javascript, avoiding the use of 3rd party deps to parse currently supported output format, and offers better performance as well.

Example command:

lntopo-cli timemachine restore --fmt=json ~/Downloads/gossip-20210908.gsp 1521846000 > topology_1521846000

Example output (after formatting):

{
  "directed": true,
  "multigraph": false,
  "graph": [],
  "nodes": [
    {
      "id": "02d97e94cfeedca2a3da47acb400bc6836e671b3cb3fc05bdd6993acd64483d09a",
      "timestamp": 1521331315,
      "features": "",
      "rgb_color": "3399ff",
      "alias": "02d97e94cfeedca2a3da",
      "addresses": "",
      "out_degree": 1,
      "in_degree": 0
    },
    {
      "id": "0246dae073f563308943c44f4d072f226f4968ea6a5a82079e9bfc9860a8a620f9",
      "timestamp": 1521812723,
      "features": "",
      "rgb_color": "f0a000",
      "alias": "pokatoo",
      "addresses": "ipv4://149.202.71.189:9735",
      "out_degree": 1,
      "in_degree": 0
    },
    {
      "id": "032c4b954f0f171b694b5e8e8323589e54196b48cf2efc27692513a360cb11d76f",
      "timestamp": 1599830749,
      "features": "",
      "rgb_color": "56a580",
      "alias": "nute.net",
      "addresses": "ipv4://79.137.115.56:9735",
      "out_degree": 0,
      "in_degree": 1
    },
    {
      "id": "03fab7f8655169ea77d9691d4bd359e97782cb6177a6f76383994ed9c262af97a5",
      "timestamp": 1620177588,
      "features": "0aa2a1",
      "rgb_color": "83b166",
      "alias": "webworker.sh",
      "addresses": "ipv4://158.69.255.49:9735",
      "out_degree": 0,
      "in_degree": 1
    }
  ],
  "adjacency": [
    [
      {
        "scid": "513887x1177x0/0",
        "source": "02d97e94cfeedca2a3da47acb400bc6836e671b3cb3fc05bdd6993acd64483d09a",
        "destination": "03fab7f8655169ea77d9691d4bd359e97782cb6177a6f76383994ed9c262af97a5",
        "timestamp": 1521522116,
        "features": "",
        "fee_base_msat": 1000,
        "fee_proportional_millionths": 1,
        "htlc_minimim_msat": 1000,
        "cltv_expiry_delta": 144,
        "id": "03fab7f8655169ea77d9691d4bd359e97782cb6177a6f76383994ed9c262af97a5"
      }
    ],
    [
      {
        "scid": "514798x115x0/0",
        "source": "0246dae073f563308943c44f4d072f226f4968ea6a5a82079e9bfc9860a8a620f9",
        "destination": "032c4b954f0f171b694b5e8e8323589e54196b48cf2efc27692513a360cb11d76f",
        "timestamp": 1521810524,
        "features": "",
        "fee_base_msat": 1000,
        "fee_proportional_millionths": 1,
        "htlc_minimim_msat": 1000,
        "cltv_expiry_delta": 144,
        "id": "032c4b954f0f171b694b5e8e8323589e54196b48cf2efc27692513a360cb11d76f"
      }
    ],
    [],
    []
  ]
}