[Elastic Agent] No ES output validation can lead to stopped sub processes and no logs

The `Fleet settings` UI allows for setting all kind of values in the `Elasticsearch output configuration`. There is no validation, allowing for any kind of input.

## Observed behavior:
* Setting `enabled: false`: the Elastic Agent kills the sub processes and only logs that it stoped the sub processes. No indicator why they are stopped, no additional entries in the sub process logs.
* Using wrong type, e.g. `bulk_max_size: "4s"`: Elastic Agent keeps restarting the sub processes, which can't start because they all have an invalid configuration. Subprocess logs include detailed information - helpful for debugging

## Expected behavior:
* the logs should always contain helpful information - if the Agent stops the sub processes, a reason should be logged.

Ideally there would be some validation preventing to store invalid configurations
* only supported options could be allowed
* _loose validation_ - (most important) supported options are validated, other options are passed on but not validated

I wasn't certain whether this belongs here and/or to Fleet.