Make log function customizable

Hello 👋🏼

In some cases (multi-cloud deployments) logs are aggregated through a 3rd party service or they need to go through a client library to be streamed.

It would be nice to replace `logpoint()` with something at configuration time like this:

```js
require('@google-cloud/debug-agent').start({
  logFunction: (message) => myRemoteLogger.debug({ txt: message, foo: 'bar' })
})
```