Allow setting user_agent

## Is your feature request related to a problem?

`user_agent` is a standard transaction field, but I can't set it via the public API: https://www.elastic.co/guide/en/apm/server/current/exported-fields-apm-transaction.html#_user_agent_4

## Describe the solution you'd like

```java
Transaction transaction = ElasticApm.currentTransaction();
transaction.setUserAgent(...);
```

## Describe alternatives you've considered

I'm not aware of any alternatives right now, except for #433, but that's still open, too.