Include height for spent outputs in get_outputs api

[antiochp]

get_outputs api returns both spent and unspent outputs.
These are retrieved from full blocks so we know the block height for all of these.

This PR ensures spent outputs have their block_height included in the api response.

Resolves #3399.

---

Block height included in all outputs, both spent and unspent -

{
  "id": 1,
  "jsonrpc": "2.0",
  "result": {
    "Ok": [
      {
        "block_height": 793277,
        "commit": "09740105055d127b6d899681ba61bfd538b6364028f044f7a1af144fc5ee78da9f",
        "merkle_proof": null,
        "mmr_index": 0,
        "output_type": "Transaction",
        "proof": null,
        "proof_hash": "6360c4fc371aa1eda21446692ab2eda1408705a47fa90117412afe70b621df0e",
        "spent": true
      },
      {
        "block_height": 793277,
        "commit": "0970e4fd62b2a84ed618955e4608da137aee1c99472db61509c3f31bb41dc5ea28",
        "merkle_proof": null,
        "mmr_index": 7851400,
        "output_type": "Coinbase",
        "proof": null,
        "proof_hash": "9490768ffb58bf1fbdc1aab0c6797f2bad333696b59befb088a77d79a3c55a45",
        "spent": false
      },
      {
        "block_height": 793277,
        "commit": "09aa84daa93505ccd47199b1cd34e967b76577f1654246207e3b1e5bd4846ea3b3",
        "merkle_proof": null,
        "mmr_index": 0,
        "output_type": "Transaction",
        "proof": null,
        "proof_hash": "6ef952d5d96390a7abf856dd5f5963f45d1c9af89a0765663c445b9d60c2a88b",
        "spent": true
      },
      {
        "block_height": 793277,
        "commit": "08147203b217e7b0d01857d83ec50cdaa72f572d273ccd743e12c6f4a51f439dee",
        "merkle_proof": null,
        "mmr_index": 7851404,
        "output_type": "Transaction",
        "proof": null,
        "proof_hash": "371d201ac886ea9e5c8e7e17e14b0fce2229e37c7b363a930eb9ab76e1e499a5",
        "spent": false
      }
    ]
  }
}

[quentinlesceller]

Looking good LGTM.