Livepeer should identify itself in `user-agent` HTTP header

**Describe the bug**
Currently, outgoing HTTP requests (at least from the webhook) have a generic go library as its header. I'm getting:

```
'user-agent': 'Go-http-client/1.1',
```

For a variety of reasons, we should identify ourselves — we already have the version baked into the binary, so we should just be `livepeer/${version}`.