This PR builds on #17283 (now merged) and is followed by #17585.

It modifies the value returned by rpc getaddressinfo `labels` to an array of label name strings and deprecates the previous behavior of returning an array of JSON hash structures containing label `name` and address `purpose` key/value pairs.

before
```
  "labels": [
    {
      "name": "DOUBLE SPEND",
      "purpose": "receive"
    }
```
after
```
  "labels": [
    "DOUBLE SPEND"
  ]
```

The deprecated behavior can be re-enabled by starting bitcoind with `-deprecatedrpc=labelspurpose`.

For context, see:
- https://github.com/bitcoin/bitcoin/pull/17283#issuecomment-554458001
- http://www.erisian.com.au/bitcoin-core-dev/log-2019-12-13.html#l-425 (lines 425-427)
- http://www.erisian.com.au/bitcoin-core-dev/log-2019-11-22.html#l-622

Reviewers: This PR may be tested manually by building, then running bitcoind with and without the `-deprecatedrpc=labelspurpose` flag while verifying the rpc getaddressinfo help text and `labels` output.

Next steps: deprecate the rpc getaddressinfo `label` field (EDIT: done in #17585) and add support for multiple labels per address. This PR will unblock those.

ninja all check check-functional