[Agents] Align on server timeout

All agents should have an `ELASTIC_APM_SERVER_TIMEOUT` config option that controls the timeout when communicating with the APM Server (see the [agent config option alignment spreadsheet](https://docs.google.com/spreadsheets/d/1JJjZotapacA3FkHc2sv_0wiChILi3uKnkwLTjtBmxwU/edit#gid=0)).

Currently this config option isn't aligned across agents. Here are the unanswered questions we need to discuss:

1. Should streaming and non-streaming agents measure the timeout differently, and therefore have different default timeout values?
1. What should the default timeout value(s) be?
1. Do we also need a separate socket connect timeout, should it be configurable by the user, and what should its default value be?

Let's discuss these questions below and update this description once we reach a conclusion 😃

### Conclusion

_TBD_

### Implementation checklist

_Waiting for conclusion_