Fields param is missing for changesreader.get

## Expected Behavior

[The CouchDB documentation](https://docs.couchdb.org/en/stable/api/database/changes.html#selector) describes that it is possible to provide _selector for `_chages` handler
```
POST /recipes/_changes?filter=_selector HTTP/1.1
Content-Type: application/json
Host: localhost:5984

{
"selector": { "_id": { "$regex": "^_design/" } }
}
```

[The documentation also describes](https://docs.couchdb.org/en/stable/api/database/find.html#filtering-fields) that selector can contain a list of specified fields to filter.

```
{
"selector": { "Actor_name": "Robert De Niro" },
"fields": ["Actor_name", "Movie_year", "_id", "_rev"]
}
```

## Current Behavior

Right now it is only possible to provide the `selector` parameter, but not the `fields` parameter. [Link](https://github.com/apache/couchdb-nano/blob/c84d0a43b13edc888a2a339f7fb057ad66d3d3fe/lib/changesreader.js#L168)
```
if (self.selector) {
req.qs.filter = '_selector'
req.body.selector = self.selector
}
```

## Possible Solution

Locally I've fixed it just by adding another parameter. It can be something like this:
```
if (self.selector || self.fields) {
req.qs.filter = '_selector'
req.body.selector = self.selector
req.body.fields = self.fields
}
```

It also may worth adding another custom `body` param similar to `qs`, which adds additional body params.
```
Object.assign(req.qs, opts.qs)
// New
Object.assign(req.body, opts.body)
```

## Steps to Reproduce (for bugs)

It should be straightforward to reproduce, but I can add exact steps if needed.

## Context

I was trying to fetch few fields from https://replicate.npmjs.com, but I didn't need all extra fields as versions, because it noticeably slows the script down and increases network consumption.

[More context](https://github.com/npm/registry/blob/master/docs/REPLICATE-API.md)

## Your Environment
* Version used: `"nano": "^9.0.3",`

Start in lib/changesreader.js around line 168 and trace how changesreader.get builds the request when selector is provided. Verify the documented fields option and the proposed body-parameter behavior against CouchDB, then confirm that a fields list is sent for filtered changes requests without changing existing selector behavior.

Written by the indexing model from the issue text.