plugins.twitch: new plugin command --twitch-api-header
plugins.twitch: new plugin command --twitch-api-header

- allow user header for api http requests
- can be used for `Authorization` or other headers
- change PlaybackAccessToken playerType to `site` as `embed` will show ads
- remove some unsed HLS and headers code

---

```
streamlink https://twitch.tv/twitch best \
    --twitch-api-header "X-Device-Id=twitch-web-wall-mason" \
    --twitch-api-header "Device-Id=twitch-web-wall-mason"
```

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