Provide entire beats config file in environment variable

**Describe the enhancement:**

I would like to be able to provide the entire Metricbeat config file in a single environment variable rather than having to manage and mount a separate file into a docker container or build the config file into the docker container and rebuild the container for any configuration changes.

**Describe a specific use case for the enhancement or feature:**

Due to limitations of Elastic Agent Integrations not supporting all Metricbeat configuration options, we are needing to run Metricbeat as a sidecar in AWS ECS Fargate to retrieve metrics from a Prometheus endpoint. Ideally we wouldn't have to build the config file into the docker container or rig up some terraform to upload the config file and we could just pass it inline to an environment variable that Metricbeat can use directly.

As an example, we run Atlantis in ECS and it accepts a config file in json format as an alternative to mounting the file.