beater: add support for reloadable config

[axw]

In #4228 we added minimal support for running apm-server under Elastic Agent in standalone mode, by waiting for config to be received once. To support Fleet completely (as part of #4004), we'll need to support dynamic config reloading.

We should look into using libbeat/cfgfile.RunnerList, which takes care of starting and stopping "runners" in response to config changes. This should be good enough to start with. Eventually we may want to handle config changes without a full restart of the server.